A clam is a type of bivalve (so are oysters, mussels, scallops, etc.). When they are 'startled', they typically clamp their shell together (like two doors closing together). They have rudimentary eyes, so they'll 'startle' at sudden lights/shadows. Depending on the size of the bivalve and where their mantle (the part with the eyes that opens and closes) is facing, you might not be able to see much of a reaction. If it's dead, you won't see any reaction (a lot of bivalves don't usually do well in our aquariums, so there is a chance it may be dead, but others do fine).
JojosReef's suggestion above is good too. The siphon it would feed with might not be hugely noticeable, but, again, assuming it's alive, you should be able to see some sort of opening/closing response.
