Here is a brief explanation of how to test SpamAssassin to make sure it is working.
During installation
If installing from source, before running "make install", run "make test".
After installation
Send yourself GTUBE, the Generic Test for Unsoliticed Bulk Email:
http://www.spamassassin.org/gtube/
Due to a bug in some versions of SpamAssassin, don't send it using your own account if you are using the Auto-Whitelist. Send it using a throwaway account or just delete the auto-whitelist file after sending the test.
If SpamAssassin is working, the message will be marked as spam. Note that SpamAssassin only marks messages as spam and that how it does this marking may be affected by your front-end. If you're not using the "spamc" or "spamassassin" program and are using a third-party front-end (especially amavisd), then it may not be completely obvious how spam messages are handled.