If your tires are less than 10 years old, they should have the number 622 printed on them somewhere. If they do, then you're golden. Any 700c (by 30 to 40) tire should fit on your bike depending on tire clearance.

If they are over 10 years old, buy a new set of tires anyway. Look for something with a reasonable puncture resistance rating. Schwalbe Marathon, and Panaracer Tourguard tires are good rugged tires. There are also lots of choices for lighter durable tires.

A local bike shop can help verify tire sizes.