One of the easiest ways to spot a self-published book instantly is to look at the interior margins. Not the top and bottom margins, they typically seem to be okay. But the margins on the left and right hand sides of each page. Again and again we see books with tiny, tiny margins. Perhaps the author/self-publisher is trying to squeeze as much text as possible on each page to lower the page count to then lower the printing bill. Or perhaps the self-publisher is simply not aware of the importance of margins. Remember, it may look great coming out of your home printer but that's not necessarily how it will look once it is bound into a book.
So, you ask, just what should my margins be? The minimum width is 1/2" but we'd suggest a larger margin, up to 3/4". Better yet, take a trip to the local book store and check out the margins in some of the books in your genre. Even better, hire a designer who will give your book a professional look and get those margins correct!
Try this book margin calculator. It's fun to play with and will give you a start on correctly setting your margins.