How to determine amplifier class (A, AB, D, etc) when not specified in product info

Hello,
I'm fairly new to speaker amplifiers but I understand the different classes (i.e.: Class A, Class AB, Class D, etc). But how would I identify the class of an amplifier, for instance if I was buying an amplifier online and the class was not mentioned (which often seems to be the case, from my experience), how could I know the class of the amp?

See amplifier classesWithout having the schematics to hand it could be difficult to say what is being employed if the information is not supplied. Many low voltage single rail audio amps these days are probably switching, so class D or above. High quality amps with low distortion and high power output, and therfore higher power supply rails will usually be A through C.Cheers,Richard

Class C is a switching amplifier where the switching clock is the signal, and the output circuit is tuned to the clock frequency. Class C was used heavily in high-power vacuum-tube radio-frequency transmitter output stages. Output amplitude was modulated by varying the power-supply voltage, if amplitude modulation was needed. Phase or frequency modulation was achieved by modulating the clock phase or frequency. On-off keying was achieved by gating the clock or by gating the tube's DC cathode connection.

I  used class C in the old days to  frequency multiply an RF oscillator, got reasonable results at x3, quick and dirty but it worked,cheers,Richard