how do i know if my caliper is seized?

just as the title says, how can you tell if a caliper is seized? what tests can you perform, or what should be noticible? i have a situation right now where my rear driver side break pad is totally worn down to the metal, leaving me no choice but to replace the rotor. i want to know if i have to replace the caliper as that might be the root of the problem. where can you buy a replacement caliper from and how much do they usually run?