Various ways of finding out what edition a book is.
The first place I look is AddALL
http://used.addall.com/ If that is not fruitful, I use WorldCat. [You can log into WorldCat through you library. If you can log into your library from home, you can then log into WorldCat from home.] WorldCat lists all the books from the member libraries around the world. The info given on a book is generally good enough to tell what edition a book is.
Note, it is not always true that a first edition is substantially more valuable than a second or third edition.
Actually, I have one book that has three "first" editions, and, truthfully, each has claim to the title.
What a great resource! I've been trying to find copies of one of my Dad's books that's been out of print for quite a while and lo and behold...I found more than I can imagine!!! Thank you!!!!