This really depends on the amplifier. Back in the early days of solid state, you'd often find DC offset adjustment instructions in service manuals and they would specify a range of acceptable values to aim for. I'd expect these instructions to call for 10mV or less. With more modern solid state HiFi amps, the entire circuit will be directly coupled and there will be DC feedback that keeps the output of the amp at 0V. If you have such an amplifier with DC on the output, that can happen due to shorted output devices trying to deliver rail voltage to the speaker jacks. Likely a protection relay or a fuse will prevent that from actually happening.
To measure this, you would usually measure the DC voltage present at the collectors of each NPN and PNP device in the amp. Another safer measurement is to simply measure for continuity between pairs of output device legs with the amp powered off to look for shorted output devices.
