How pessimistic, Juha! I ride a lot and I have a number of perfectly good tires that I wouldn't hesitate to donate to a poor friend in need.
Careful inspection is the key here. Obviously, feeling the inside of the tire for pieces of glass, metal, or other sharp things is a start. Try to find spots where the fabric inside the tire is showing through the rubber - those are bad. So are large cuts; small puncture holes aren't so bad.
When you mount the tire, inflate it to maximum pressure and look for any bulges - if there are, throw that tire away.
I'm sure your friend can help you inspect the tires he has to give you... Read what Sheldon Brown has to say about
tires. And good luck!