Root cause of warped rotors?

[bobs]

So, for the second time in 5000 miles, my '97 S420 has warped its right front rotor. I know that the two most likely causes would be a sticking caliper or a collapsed rubber brake line, so my question is how do you make the diagnosis as to which one it is?

I know I already have to spring for another rotor but I'd rather not have to buy a caliper too if I can at all help it.

Thanks,
Bob.