If the existing amplifier (gold box) shows signs of the potting compound on the rear to have been breaking down and "melting", then it's bad. That is the most common indicator of failure or intermittent operation due to heat build-up.
There is no dedicated test procedure for the amplifier itself per se. Extensive test procedures of the system are covered in the appropriate ihc service manual, in order to do that, a special "test switch" device is needed which is not available unless someone has one obtained from an ihc service dealer at some point.
I simply substitute a known good amplifier if I suspect a gold box is in failure or has failed.