If you are lucky enough to be born with "Popeye" forearms, then perhaps you can get away with not training them. You can be getting the fringe benefits from other body parts being worked in conjunction with them. But if that is not the case, then it behooves you to do some isolation exercises.
You can do these with either a barbell or dumbbell at the end of a bench. Use a full range of motion and a weight that you can handle properly for 12-to-15 strict reps. The barbell sets go quicker, obviously, since you don’t have to alternate each arm. A great movement for your wrist flexor muscles.
REVERSE WRIST CURLS
Standing with a barbell behind you, these can actually give you a slightly longer ROM at the top of the rep than the regular wrist curls can.
Use a barbell and these will work the forearm flexor muscles well, giving you some nice detail in a very conspicuous spot.
Yes, these are truly a biceps exercise but you will really isolate the brachioradialis muscles, which are part of the forearm.