How is the power supply keeping up?

You could grab the multimeter, but you could also buy one of the surprisingly cheap V20D voltmeters from Evilbay, deal-and-scream or Mines-in-a-box.

Mounted on the printer or the power supply, you will have a constant read-out of the voltage.
(And LEDs are great and make you happy!)

It is cheap for a reason. It is just three 7-segment LEDs and a bare PCB on the back.
It deserves a casing, and here it is:

It is easy:
Print the two parts, clean the holes in the lid with a 3mm drill, in the casing with a 2.5mm drill and maybe you need to file the notches a bit to fit the PCB (depends on the printer).

Put it all together and stick it somewhere appropriate.
(Double adhesive tape is great).

• Black wire goes to ground.
• Red and white goes together into the positive connection.