Sometimes you can see the worms in the poo. Most vets will do a fecal float test and tell you what they have and what to treat it with. Or if you feel you have sen worms, you can get wormer from the feed store -- but I am not familiar with what to use -- seems to me Ivermectin is supposed to get most worms, but I could be all wet here.
If they are 5 years old and you have never wormed them, it is a good possibility this is the problem.