The best way, is to have a fecal sample done, so you know exactly what to worm for.
A general way, if you do not believe them to be over infested is to use an otc wormer containing both Ivermectin and Praziquantel. Then use these different drugs in rotation.
Ivermectin-effective for large/small strongyles,pinworms,ascarids,hair worms,stomach worms,bots,lungworms and threadworms.
Praziquantel-tapeworms
Benzimidazole-large/small strongyles,pinworms
Moxidectin-large/small strongyles,ascarids,pinworms,hairworms,stomach worms,bots
Pyrantel-large/small pinworms,large roundworms.

I de-worm every season change (4x a year) Some people de-worm every 4 months, others every two, and so on.
I like to rotate wormers, just as Ltc4h said, different wormers kill different types of worms.

The only thing that I would add to what Ltc4h, & AQHA had to say is that if your horses are on pasture during the spring and summer and some point I would double dose them with Strongid (a pyrantel pamoate) for tape worms. Most single ingredient dewormers have a good safety margin (ivermictins, pyrantel pamoate, fenbendazoles). You have to be a little more careful with some of the combos and Quest as they don't have as large of a safety margin. But there are many to chose from, just check the active ingredient and rotate them.