I'm using Eclipse and Java, but the answer doesn't need to be specific to those. I'm sure platform specific answers will get plenty of upvotes if they're far quicker than non-platform specific ones.
Right now I'm using
grep "classname"
find . -regex .*\.java
> uses_classname.txt
to find code that might be affected by a change to the class. If there are only a few lines returned by this, I can look at it manually as it is. Otherwise, I can grep again to find the specific methods I'm modifying. Is there a better way to do this?