Grohe cartridge replacement?

I have a leak in the area of a Grohe shower valve. Is there a way I can confirm it's the cartridge before I replace it? This valve was purchased/installed in 1994, so no warranty replacement. The leak is very slow and seems to be worse when my son showers, I can't figure out why. Could it be anything else than the cartridge? Is there a way to visibly check it?

Thanks!

Take the handle and trim off, then install the handle so you can turn things on. You might try calling Grohe, they may send you a new cartridge for free. That policy changed over the years, and yours may be before, but it doesn't hurt to ask.