Power Supply

Much depends on what you wish to use the power supply for. However, try a 10 ohm 50 Watt resistor across its output and measure the D.C. output voltage for purity from A.C. ripple and/or noise. A scope would be nice here.

If you have the specs for this power supply, does it conform to your expectations? Maybe some of its caps have aged and require replacement?

A 24v truck headlamp bulb or two is a good test load for this sort of thing. You can put just one on the end and sweep the supply from 5 to 20v whilst monitoring the output for voltage (make sure that it goes as far as it did with no load) and for ripple ('scope useful here, but low AC voltage range on a multimeter will do), then set back to half way - say 10v - then put a second bulb across the output to double the load, and make sure that the output voltage does not drop by more than a few mV. At the midway point of

Arfa

How about providing a less open-ended question. Some details would be nice. Otherwise you may get answers like: To test the dimensions, use a ruler. To test the weight, use a scale. To test the voltage, use a voltmeter.

I'm impressed that you got two good answers already!