You can't know that you will be until you try
Unfortunately, the way you know you can be John Grisham (short of a DNA test) is by dedicating yourself, and years of effort, to getting published; at which point you'll be able to find that book of yours on a book shelf, confirm the text inside of it is yours, and that the glossy name on the front page is indeed John Grisham. That slog you're looking at and feeling like you don't want to approach isn't really optional. However, if you want to know if you're good enough right now the answer is pretty simple. Take that book you wrote, revised, edited; you know, the one that was approved by alpha/beta readers, and try to sell it.
You did write a book, didn't you? Oh, you haven't. Well, then it's impossible to know whether you can be a successful author. Because the only thing that really at the end of the day separates a "successful" author from a person who just writes is how many books they've sold and how many people appreciate those books.
You might be able to tell that you are not ready
Ok, well, if you haven't written a book then you've at least written a short story? Ok, let's just assume you have. Submit it. I don't care where, but submit it. Send it in. Get someone's honest eyes from your target market or industry on it. Don't ask a friend. Don't ask a relative. They will lie to you because they love you (or fear you, it's all the same really). Submit it to someone you don't know. If they don't write back, you are not there yet.
Ok, non-novice reader, you've read the above thought, "1 person? Did he write 1 Person!? WTF! That's awful advice! I'm sure he meant submit to 200 people."
Good catch, sir. I did in fact mean 200 people. But if you are a novice and you're reading this and you balked at submitting to 1 person, you're not ready yet. If you submit and get feedback for improvement or any response other than a silent rejection you know you have a chance; a small one, but no one spends any time responding to bad work. If they requested revisions, then you are closer to making it than 99% of people who pick up a pen. Good job. Now, get back to work. If they, by the grace of John Grisham, accept your work. Congrats, you've got talent kid. Maybe, just maybe, you're John Grisham.
There are certain learning difficulties that might stand in your way. I don't want to go through the list because there's a pretty good chance I'll offend someone; and I don't know what that exact list is; and... it's not just a disability thing, you may be a capable individual with personality defects... The point is... writing a book does tend to require the ability to take varying perspectives and exercise a multitude of sometimes innate, sometimes learned abilities. If you've found mastery of the written word difficult up until this point in your life, you can bet that it's not going to get much easier if you've never been able to improve (supposing you've previously given it a valiant effort and had the benefit of good teachers). Writing can be taught, certainly; I would never say the opposite. But there is a sort of baseline, a min-capability and most people aren't below that line; most people also don't really try to improve their position with respect to that line. If you suspect that you are below that line, you could reach out to a specialist in learning disabilities to acquire some strategies, maybe even to ask for advice. No one will be able to tell you for sure that you can't be a famous author; but if you're in this category it will most certainly and unfairly be harder.
If you sit down and you can't stay focused when you go to write, and a couple of weeks of effort at trying to write doesn't show any improvement, you likely need to focus on your executive functioning skills and attention. The good news is that this is better for you generically and won't just help your writing life; but if you can't maintain that focus, you can't rightly expect to maintain your attention long enough to finish a book. A book is a marathon, one which requires well formed muscles: those muscles are attention and fortitude. I'm not saying you have to pull yourself up by your own bootstraps. Frankly, that doesn't work. If you pull on your boot straps, the best you can hope for is to not fall over. John Grisham may be a great writer, but he hasn't redefined the laws of physics. Like John Grisham, you must work within the realm of physics, and also you need to able to consistently and regularly perform.
You don't take criticism well
Look, almost no one is ready to publish a book from the outset. If you find you don't get along with people; If you find that you can't learn from other people when they point out your deficiencies; If you find you don't like the kind of interaction where other people tell you how bad you are at things; it is unlikely that writing is going to be your meal ticket. You can probably learn to cope with deficiencies in this area, even get better, but every successful writer is either hard on themselves or has an external someone to do that. If you can't find the constructive element in criticism or can't form working relationships then you likely will not be capable of writing a book. You'll probably also be bad at many other things in life.
If you can't generate story ideas that intrigue others.
Chances are, as a novice, you're thinking of a story someone else has already told. Maybe it's about a young boy who gets told he has to go on a journey and face some great evil, that person who tells him this thing eventually dies after mentoring the young boy into a promising man and then that man must face down evil. Well, it's certainly possible you're doing this intentionally, but if you weren't and I just described your story there's a very good chance you haven't done a enough research into the craft of writing yet and you are telling a story that already exists. Almost every male author who grew up reading fantasy writes a story about a young boy and his dragon. And that book is usually awful and unpublishable. (I wrote one, I'm not unique)
But here's the thing. If you can't come up with a story and tell it in about a sentence and get other people excited then you probably aren't there yet. This is called the pitch; it and various other summarizing techniques are a requirement for becoming famous. You must be able to summarize your work, you must be able to get other people excited about said work. If you can't, then likely can't sell. You can improve your abilities in this area, but it's basically a requirement.
If I could tell you if you could be an author, I'd have a $1,000,000
Probably more. Think about it. You're asking us to predict the future. Do you know anyone who can do that? Do you honestly think that if it were that easy anyone would make it out of high school without a signed contract like a Basketball player? If you could lock down every famous author out of high school you would own the publishing industry. Since no one has done that, it's probably safe to assume that no one has cracked the formula yet.
The other thing you might want to be aware of, since you're already skittish, is that 80% of writers don't make a living on novels. That's probably a low percentage. People write articles for newspapers and it's not great money, but its money. People write ads. People write manuals for how to use computers, machinery or software. None of that is glamerous, but people make money doing it. Most authors who get published get advanced $2000 (if they are very lucky) and never see another dime.
If you're doing this for the fame, if you're doing this for the money, you're probably doing the wrong thing. Go work out, take some acting lessons and get some plastic surgery; it'll probably work out better.