I sometimes look at the number of chapters…usually they re-divide the book on abridged versions. It can be really hard to tell sometimes! If you already have the book, it often says on the back of the titled page. It usually says unabridged or names the person who edited the abridged version.
Currently, I like to buy Sterling Classics. They are beautiful, hardcover editions, all unabridged, and very inexpensive. Rainbow Resource carries them for $7.75 each, which isn’t much more than a paperback copy or a used copy by the time you factor in shipping! Many of the SCM read aloud books are available through them.
Dover usually has unabridged copies for an inexpensive book, but they aren’t the best quality. We often use Dover books for our poet studies since they are so inexpensive.