NJT,
There is not likely going to be any visual indication of the problem. However there are some logical steps you can take to isolate the issue to a component, then to a particular tube type and potentially even down to a single tube.
First step would be to isolate the issue to either the pre-amp or amp. To do this, simply cross over the interconnects between the amp and pre-amp. So left pre-amp output to right amp input. If the problem moves from the left channel to the right channel, you know the problem is in the pre-amp. If the problem does not move from the left speaker to the right speaker, you know the issue is in the amp.
Once you determine what component is causing the problem, you can systematically swap tubes betwwen the left channel and right channel. I would start with a single pair of tube, like the output tubes or the input tubes.
I'm not an expert and my tube experience is mostly limited to components that have a relative small number of tubes. Hopefully you will be able to isolate the issue relatively quickly, then decide if you need to re-tube the entire component or just replace a single tube or tube pair.
Enjoy,
TIC