Without a lot of experience in the field, I'll give following advice:
Give it time and have beta-readers.
Here I'll focus on the readers.
A great way to check whether or not something has enough value to be thoroughly reworked is to have beta-readers.
Some, that have read the work without talking to you about it first.
Some, who have read it, knowing what you want the story to contain, express and evoke in the reader.
Sometimes, the readers will give you the same feedback.
Most of the time, they will focus on wildly different things, because previous knowledge changes our perspective on things.
So, the first thing is to have some people read it.
People who are close to you may not be completely honest with you because they do not want to hurt your feelings. They may not even realize that they are sparing you of the truth. If you want feedback from them, tell them to be brutally honest - but expect it to be rough.
People who are not close to you will be more honest. Finding such people to read something "unfinished", is probably not easy either, though.
If you're lucky, you find a few people who can communicate to you what they feel is lacking or unsatisfactory in your work, in a way that makes sense to you. If they are also willing to re-read it, that's truly a gift. Establishing such a relationship with someone is probably most realistic if they too feel that you 'give them something in return' and that they're valued.
This could be the case in writing groups.
I have no experience in this (yet), but I guess that's most of the reason they exist.
In this case, you could have some standard questions that you ask your readers. For example:
- What was your experience when reading the first 5 pages?
- What is your understanding of the things that happen to x character?
- What did you think of the way things ended?
These three broad questions could have their own sub-questions. Those questions could get more and more specific.
Perhaps having readers read smaller parts of the story will make your questions easier to answer.
An important tip is to give the reader as open questions as possible, eg. don't ask questions with yes/no answers, unless that is exactly what you need.
The actual signs
If you end up with wildly different answers to all the questions, it is difficult to know what to change or how. In this case, you'd probably benefit from more specific questions.
If your readers more or less agree, then it should be 'easier' to find the issues within your work.
Then, it's a matter of figuring out "How to write a gripping beginning", "How to write a satisfying ending" and so on, and applying it to your specific story.
Sometimes, this is straightforward. Other times it's a serious challenge.
In any case, giving it time and thought is valuable, but feedback is crucial.