How can I tell whether a name is too difficult to read?
I wouldn't worry as much about whether a name is difficult to read; I would worry about whether it's easy to distinguish from the other names in the book. If you have one character whose name is a string of strange characters and all the others use the Latin alphabet, readers will make up their own pronunciation for that one and all is well. If, on the other hand, you have three characters named Saþil, Såþil, and Sæþil, you will confuse readers. Focus on making the names different rather than on making them readable.
How can I make names easier to read?
Transliterate them or use nicknames.
"My name is [insert gibberish here]. In your language, it would be SharpTongue. You may call me that."
"My name is [insert gibberish here]."
"I'm sorry, but I can't say that. How about if I just call you Susan?"
How can I better hint at correct pronunciation?
The tried and true method is to have someone mispronounce the name in dialog and have the character correct them ("I know it's spelled Shithead, but it's pronounced sha-THAYD"), but as Phil S said, that gets old pretty quickly.
You can also include a parenthetical or footnoted pronunciation the first time the character is introduced. That's arguably less intrusive than the mispronunciation route, but it works for people that need it.
One of my favorite methods is to insert a dramatis personae (cast of characters) at the beginning of the book that includes the pronunciations and a short description (e.g., "Eldest child of the king") for each. In complex books, I always appreciate those (and often bookmark them while I'm reading), and people who don't care will just skip over it.
Yet another idea is to create a book trailer. Put it on the book's website, YouTube, and whatever social media you use. Direct people to it in the introduction. That way people will hear the name before they even start reading the book.