As mentioned above, they are non-photosynthetic and will require both phytoplankton and zooplankton to do well - with live phyto and rotifers being a good choice. I have seen them live for surprisingly long periods of time (over a year and counting). From what I have read they only live 3~4 years in the wild, and since we are obtaining them as adults they will probably not live much longer, at best, than a few years.
When healthy, they will be strongly attached to a surface by the use of filaments that originate near the base of the shell. The shell should be open, and the flesh should appear healthy and not retracted or discolored. They seem to prefer darker recesses that are out of the way. I have never tried spot-feeding one, but it would seem helpful.