There are several sites that offer this information. My favorite is crosswordgiant.com - when you enter an answer it displays all the occurrences of its being used, sorted by the clue. You can then click through to see all the answers and clues from that particular puzzle, which makes it fairly easier to see the theme and determine if its the same or similar to yours. This site covers a wide range of publishers.
This site cruciverb.com has a similar feature (Puzzle Database). Here you can actually see the grids (not just a list of answers/clues). I believe you have to have a paid membership to use this, and it only covers a limited number of publishers and isn't always up to date.
xwordinfo.com has an excellent database, but it only covers NYT.
Crosswordtracker.com will show you if your theme entry has been used, but you can't see the rest of the puzzle. I use it a lot because it's very fast and compact and displays occurrences of the entries in chronological order, which is helpful.