It is quite straight forward to check the resistors.

1) Unplug from power
2) Lie them flat on the front (carefully)
3) Undo the screws holding upper part of the back panel
4) Gently lift until you can peak in and see the condition of the resistors. Be careful there is the power connector at the lower end and the display connector at the top and the connector to the speaker drive units in the middle
5) If you want to take a closer look the three leads mentioned above will need to be disconnected.
6) Refitting is the reverse of the above...

This won't tell you the condition of the PCB underneath but will give you an idea.

Lounge: DSP5200, 518
Snug: DSP33 x 5, C61R, HD621
Office: M33