>>>Generally if you have no ALC showing on your radio you will not be over driving on PSK. This will vary somewhat by radio. The best check is to ask another station to check you signal.<<<
I would go one step further and recommend that you ask several stations over the course of transmitting on several different bands. Short of having an IMD meter like:http://kk7uq.com/html/imdmeter.html
it is difficult to know for sure if you are actually over-driving the radio. As everybody has said here their suggestions are "generally speaking." There are exceptions. For instance my Elecraft K3 actually wants to see 3-4 bars of ALC because of the way Elecraft meters ALC. With most rigs *any* ALC registering is not good. In some rigs no ALC registering can *still* mean you are over-driving the radio.
Additionally the problem with any given signal report is that distortion can occur anywhere in the the signal chain - including on the RX end. You may have a good strong clean signal that introduces distortion on the receiving station's radio. It might not even be due to the fact that you are using a lot of power - it just might be that your antenna is efficient and propagation is providing a solid path to the wonky receiver.
Short of the above IMD meter, checking with a number of stations and making on-air adjustments is probably the best way to get a feel for how your signal is getting out into the waterfall. Due diligence is appreciated!
73 de Eric, KG6MZS