Pro Reverb Voltages

[dstrat]

Fender used I believe 2w resistors for years for 6L6 amps and still work fine, well unless they were abused.

Ha, After going through my old Fender schematics it seem they were using 1W screen resistors so I stand corrected.

[pdf64]

...how I would use the scope to check for oscillation, say in the context of this discussion?

I leave triggering on 'auto'. If I want to lock the triggering to the test signal from the sig gen, I switch the triggering 'source' to 'ext'. Normally just leave the source to the scope channel being used.
To identify free running oscillation, firstly it's easiest to listen for it, but if ultrasonic, just twiddle / sweep the 'volts/div' and 'time/div' controls with the amp idling, whilst checking for anything more than its regular noise floor becoming apparent on its output.
Parasitic oscillation requires a signal for it to manifest (ie the amp should be mostly stable at idle), the scope shots in Chuck's search show examples.
Bear in mind that an amp's margin of stability may differ according to its load; an amp may be stable with a resistive bench load, but instability may manifest with inductive loads such as a speaker.