How do I test my own amplifier

DC offset test is easy. Just put your multimeter in the DC mV scale and touch the leads across the speaker terminals with the amp fully warmed up and not playing anything. It doesn't matter if the reading is + or - volts, closer to zero volts the better. Depending on the amp, under 50mV is usually considered good. Some amps will have less than 5mV DC offset. If your reading is up in the 300mV DC or more, better have it checked out. Though woofers can take fairly high DC offset, tweeters cannot, but luckily, tweeters and mids are protected from DC by capacitors in the speaker's crossover.