While most dry rotting won’t be as obvious as this, it is still something to check for. If the tires are on the vehicle, roll it into a curb or something to make it flex. If they aren’t on the vehicle, let the air out and flex it yourself. Also, look for anything other than nice smooth sidewalls. Additionally, waves or bumps can indicate problems in the bead bundle of the tire. Of course, any holes in the sidewall means that most tire shops won’t fix them, and it could be illegal to drive on the road.