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?
[Just being a bit of a librarian here:]
Finding references to a Java method/class:
How can I find all the methods that call a given method in Java?
References to Java annotations:
Finding references to 'derived' methods in Eclipse:
Eclipse Specific: Is it possible to find references of use of derived methods in a class?
How to do it with VIM:
Is it possible to find usage of java classes or methods in VIM?
And with Emacs:
How can I find the references of a class, method, variable in Emacs with Etags?
Finding (the number of) references to a method in C#:
How do you programmatically identify the number of references to a method with C#
Nice to read: how to find methods that are not used in C#/.NET:
Is there a tool for finding unreferenced functions (dead, obsolete code) in a C# app?
On refactoring of PHP code:
Finding references in Smalltalk:
Finding references in (Apple) Xcode seems to be a popular question:
Find method references in Xcode