detecting channel failure in S7 modules.

[Pandiani]

Hello guys,
I have very odd problem at work. One of channels on analog input module (S7-300)went
"out of order" (sorry don't know which term to use, please suggest better, so I can
use it in the future), but there was no red LED diode indication on module.
PLC has about 20 modules (AI, AO, DI, DO, IM, CP...) in total, and proces simply started
to behave very weird. It was such problem that many things idicating that some kind of
mechanical thing is broken somehwhere in process and we lost time until we got back to PLC.
By analyzing the program (which was written by some firm, with undescriptive variable names)
and using calibrator (device that can produce constant current signal-don't know exact term in
English) we conclude that one chanel is out. By tuning signal from 4-20 mA, input variable
value simply didn't follow it. After module replacement everything was fine.
My question is:
Is there any formal procedure or way to dtermine if some channel input is bad?
We tried to read dignostics buffer in PG, but there were not any informations that would
allow us to conclude there was a problem with card.
What to do in such cases?
People who work with me (and I) are relatively young and without experience, so
any advice is more than welcome.
Thanks in advance.