I use a high-quality digital multimeter to check. You need to measure for residual ground voltage at the RCA jacks. Make sure nothing is hooked up to the component but the power cord. Try plugging it in both ways while noting the voltage reading between the RCA hole and outer ring on one channel (hole is positive). The lower voltage orientation has the polarity correct.
I did this with my vintage MC240 tube amp that had 1960s lamp cord for its power cord. One way I got like 35mV and the other way I had 6mV. I have to say I couldn't tell a difference in sound though but I am anal so I wanted it correct for peace of mind.
By the way, whenever making a low-level voltage measurement, you HAVE to use the local ground that is as close to the positive as possible. Using the outlet ground (or chassis) will do nothing but screw up the results.
Arthur
I did this with my vintage MC240 tube amp that had 1960s lamp cord for its power cord. One way I got like 35mV and the other way I had 6mV. I have to say I couldn't tell a difference in sound though but I am anal so I wanted it correct for peace of mind.
By the way, whenever making a low-level voltage measurement, you HAVE to use the local ground that is as close to the positive as possible. Using the outlet ground (or chassis) will do nothing but screw up the results.
Arthur