Do the Testing Kits expire?

The kits do expire, but last longer if kept in the freezer in the dark.

You can see if yours is still good by testing against some known substances using the document posted here:
http://dancesafe.org/node/14313

The marquis reagent is A.5 on that document, and the results for that one are on page 11.  Test an aspirin, test some sugar, and if it reacts correctly you can assume that it is still good for your other tests.