Unless its a special app better bin it and fit new, a newun costs less than a set of switching trannys.
My usual is to check the i/p rectifier, and the dc bus, then verify there is a waveform on the o/p fet or trannys, if not see if theres control volts to the smps chip, if there is then see if the osc is running, then onto the feedback network.
Honestly though if you have never messed with an off line smps then dont, they are very dangerous and you could easily end up toasted.