I agree with Simon. In many old schematics, a test point will indicate what the waveforms and voltages should be, using either a scope or DMM as recommended.
If you are winging it without such a schematic, its not easy since the test points include what should be seen give the impedance you put across the test point and ground, using the test device.
Its not safe live testing capacitors in power supplies or high voltage areas, i.e., near 50Vdc and up. In amplifier designs of different types, the schematic usually has test points.