You will need a multimeter with a DC voltage scale. There are two different types of multimeter, analog and digital meters. Below is information for both types.
An Analog multimeter is a little more complicated to use. Both Analog and Digital multimeters need to be set to the appropriate voltage, but with an Analog multimeter, you will need to choose the voltage range and must read the proper scale.
The Analog multimeter uses a needle display which moves from 0 across the scale until it reaches the voltage being tested. This multimeter has five major linear divisions with multiple scales to read a variety of ranges. An example would be three different ranges. The first is graduated in increments of 0 through 5, the second, 0 through 10, and the third, 0 through 25. Each of these ranges are subdivided into divisions that are graduated into tenths. In order to read 12 volts the 0 through 25 range would be the appropriate one.
Because DC voltage has positive and negative potentials this device is polar sensitive, this means that if you reverse the two probes when reading a positive DC voltage it will read as a negative voltage. This is actually necessary to read negative DC voltages. The two probes are differentiated by their color, Black (negative), and Red (positive). To read a positive DC voltage, the correct probes must be used with their corresponding potentials (positive to positive and negative to negative).
With the probes being used normally to read a negative DC voltage, the needle moves from the 0 to the left, "pegging" the needle. By reversing the probes you can properly read the negative voltages.
The Digital multimeter (DMM) is much simpler to use. As was mentioned previously, you will need to set the appropriate voltage. One of the advantages is that the DMM has an LCD display with a numeric readout, so there are not any multiple scales to read. Another advantage is that most DMMs are autoranging when reading voltages, which means that you will not need to set the range with these DMMs. A DMM will read both positive and negative DC voltages and display them correctly. When reading a negative voltage, a minus sign will appear on the display before the numeric value. This still is a polar sensitive device, so you will still need to use the positive and negative probes with their corresponding potentials.
To read the output of the power supply you will need to know its DC voltage, this information can be found on the back of the power supply. There will be two voltages shown, a AC voltage (110V AC in U.S. - 230V AC elsewhere) and a DC voltage (usually between 12V DC - 19V DC), you want to know what the DC voltage is.
To read these the meter will have to be set to the DC scale, with the analog meter it will need to be set to a range which will read the 12V DC -19V DC range. Insert the tip of the positive probe (red) into the center of the male connector of the power supply, and touch the negative probe (black) to the outside of the male plug and then read the voltage. The voltage readings typically will run a couple of volts higher than the posted readings, so don't be concerned about that.
If you need more help, just ask.