Not really, unless you have a friend with TCAS or ask a controller when airborne...
Or start pulling the panel apart, but that doesn't sound like a good idea.
I suggest you take off, select mode A, then as soon as you can get a radar service switch to mode C and ask the controller to verify if mode C is displaying (and matches your altitude). If not, go back to mode A: I've known ac with no mode C encoder transmit spurious altitudes when mode C is selected, which is worse than just mode A, obviously.
Tim
(PS IO540 beat me to it: but just because it says it has mode C, it doesn't mean the encoder works...)