Are you talking about PG indicators for DC-DC converters, AC-DC converters, or AC appliances?
As far as DC-DC converters go, I haven't really seen any standard in how PG detection is specified. Usually it requires that the output voltage be within some margin of the nominal setpoint (like +/-10%), and that the input voltage is within a certain acceptable range (so it has UVLO and OVLO). The exact numbers of those thresholds vary quite a bit. If you want to detect current as well, then things become much more complicated... ultimately it depends on the needs of the load you're driving. So I doubt you'll find any literature that goes too in depth into power good circuitry, unless it's quite application specific.