When checking with your stick, you're technically only allowed to hit the other player's stick and gloves. There's two basic types of stick checks: the poke check and the slap check.
A poke check is generally used when the player with the ball is about a stick's length from you. You keep a hand on the butt of the stick and push it towards the other player, letting it slide through your top hand. A poke check is usually aimed at the gloves, but you can get away with a few checks in into their chest and arms as long as it's not excessive.
The slap check is a quick swinging motion that is effective on both the gloves and the stick. Like the poke check, you can get away with the ocasional slap to the chest or arms as long as it's not excessive. You obviously can't wind up and take a full baseball swing at the guy, and any check the the helmet is a penalty.