I just bought a 85mm lens on eBay and the seller said it is in "Like New" condition. I have heard of people talking about "fog" in the lens, but how do I know if this is happening with the lens I bought?
To do this professionally:
You need to have a special piece of equipment called a lens test projector. You can take it to a good camera rental house and ask them to do you a favor and check the lens. Or mail it in to get repair, $$.
To do this on the cheap and not very scientifically:
If you have a magnifying glass or good eyes a close visual inspection through the lens in high light would probably be enough to notice element problems. remember not to look at a light through your lens and go blind okay.
You could buy or print a camera test pattern and take a bunch of photos of it then zoom in and look at them. If you move the paper slightly you will see similar distortions appear at the same spots regardless of paper orientation and position.
On a personal note, i washed one of my camera lenses through a full cycle and load of clothes. I never used it again, but a few days in rice, and a large silica pack removed almost all the moisture from inside the lens. The lens tech that looked at it had to screw it back together and when she put it on the projector it was noticeable that it was damaged by washing machine.
The answer provided by @cliffclos seems to be very good, but I also want to add some ideas.
First off, you could just ask camera repair centers, how much the check would cost. I don't think the simple check would be very expensive.
Also, for camera lenses that I buy, I like to open the aperture up and just look through the lens, preferrably against the sun or some light. This doesn't work to well for lenses under 50mm in my experience, or on small aperture lenses. Look through both sides, move the lens to look at different angles. The picture attached is a quite dusty Minolta MC Rokkor-PF 85mm 1.7. Sometimes lenses "just" appear dusty, the "cloudy"ness looks a lot like this, but just a lot finer. (Please disregard the v-shapes in the middle of the picture, they're just from the phone camera.)