When you buy them, you usually want to take the water that they came in and dump it into a smaller bowl. Then add about 3-4 cups of water from your own tank to the bowl that the clam is in. This will also raise the temp that the clam is in if it has dropped even a few degrees since getting placed in the bag (the three things that clams are most sensitive to are temp, light and salinity).
But after you are done examining the foot underneath (also try to make sure that the byssal organ is still there - you can determine this by looking for small pieces of rubble or rock stuck to the bottom of the clam), you also want to examine around the edge of where the mantle lip is. If it is a clam with scutes, the scallopy looking things that come out from the side, look into those for small snail looking things as well.
I also always like to take a toothbrush and scrub around the outside of the shell. I try not to scrub around the foot area at the bottom of the clam because the bristles could ROYALY bother the clam.