If you don't know how old the tires are, or you do and they're over about 4-5 years old, replace them. Dry-rot isn't always obvious. I pulled our son's last mountain bike out of the garage when I got started a couple of years ago. About 3 months after I started riding it, the fron tire blew out on a ride. My wife was riding close behind me. When I went down, she ran over my back wheel and bent it. The replacement wheel cost more than the tires would have, and more than the bike was worth. If I had been going faster when it blew, I could have been hurt too.