I have a grohe shower valve that has a slow leak, it only leaks for a little bit after a shower. Seems to be worse when my son showers, not sure why. Is there a way I can tell for sure it's the cartridge before I spend $70 on a non-returnable part? This valve was purchased in 1995, so no lifetime warranty.

I've got no problem changing it out (I think) - I just want to feel confident that it will fix the problem. I'm not sure what else it might be -

Update - I've taken off the drywall so I can see the back of the valve. No visible drip from the valve body, but water is pooling up in the bottom of the escutcheon. Maybe the escutcheon gasket is the problem?