First, remove the turn signal bulbs and inspect them closely for a failed element inside the bulbs. If you really can't see if one has failed, hold the body of the bulb against the negative battery post and touch the positive button(s) on the bottom of the bulb with a short wire connected to the positive battery post. Doesn't have to be anything fancy, just strip off the insulation back from the end of the wire 4-5 inches and wrap the exposed wires around the positive terminal. Strip about 1/8 inch off the other end of the wire, just enough to touch the bulb positive button(s) on the bottom of the bulb.