Electric meters are the best tool you can use to troubleshoot your furnace or air conditioner as most failures that the average do it yourself person attempts are of an electrical nature. When you purchase a meter I strongly recommend purchasing one with a CAT III 600 V rating. If a meter is connected incorrectly or not set in the correct mode it can explode. Cat III ratings mean the meter has fusing inside it to prevent meter damage and operator injuries. If it does inadvertently get connected incorrectly and the fuse blows it must be replaced with the specified fuse in order to maintain the safety of the meter. A cheaper meter has a very good chance of causing severe injuries to the novice.
The picture listed below shows a Fluke 333 meter which is CAT III 600 V rated. The top of the meter is a clamp that spreads open and clamps back together for taking amp readings. There is a thumbwheel in the center of the meter for the different testing functions. Below the thumbwheel is a window where the readings are displayed. On the bottom of the meter are 2 ports. The left port is black and the right port is red. The ports are where the meter leads plug in for taking readings.
This meter can be used to read Alternating Current (AC) Amps, Ohms, Direct Current (DC) Volts and AC Volts.