Ruler (in mm), torque wrench, 12mm socket, 22mm socket, or 30mm for the latest Monsters.
Disclaimer: As with any mechanical adjustment, if done improperly this
procedure could be harmful to your bike and could cause severe injury in
the event of a crash. If you are at all unsure of your ability to perform this procedure correctly, PLEASE do not attempt to do so.
Maintaining your chain is very important to maintaining its life. I'm not saying that you need to adjust your chain every week, because you don't. You should, however, check it before every ride. Simply reach down and push up on the chain in the middle to see that it is not out of adjustment.