First of all, pull the plug while you are still strumming and listen to the signal die out.  A lot of times this alone will drain the caps of a lethal charge BUT...NEVER ASSUME!!  6 inches or so for the leads should be plenty.  Discharging one of the power caps does it for all but this too I never assume and probe for DC at several points in the circuit.  At this point you should not have much or any residual DC above a few volts so your DMM can be set to any scale of DC really.  Leave your "bleeder" connected as caps do have a tendency to build a bit of charge even at idle.  DO NOT forget to take your discharger off before you power up or you will see smoke.

You can also wire a resistor from one of the leads in the filter cap to ground and the cap will drain automatically when turned off. Leave the standby switch to on it will help the cap drain as well. You can probe for DC voltage by sticking the black probe to anywhere in the chassis and the red probe to the leads. I had Nik do this on my Plexi 50 clone and usually there are no more than 17 volts in the system after about 3 minutes from power down.