Short version: read reviews of the work to see what other people have said about it.
One issue you are going to undoubtedly face in your effort is the fact that there are authors who will write in third person limited for most of a book and then suddenly switch to third person omniscient. It's usually bad form, but it happens.
And then there are other works, which feature a main character with a persecution complex in third person limited, and it very quickly appears like it's third person omniscient, because it talks so much of everyone thinking this or that, but it's not, it's talking about what that one character is perceiving, and that character is merely projecting thoughts onto everyone else.
Short of having a perspective tag on all books, I don't really see a way around either finding out by doing research about the book before reading it, or taking a chance.
For what it's worth, I'm hoping somebody comes along with a better answer for this one, so I'd prefer this answer not be selected as 'the' answer for at least a day.