Just to let you guys know: both of my Pentagons are up and running
Appears the first one was suffering from a flakey PSU (I was using my PC's) and C7 being soldered in the wrong place and the second one must have had a cold solderjoint, as it magically started working once I re-soldered every chip.

So ... what are good measures for testing that everything is working alright?
I've tried loading and running a demo from disk (Alienate by Hooy-Program), loading and saving a program from/to tape, testing the INT and RAM using Test 4.30 (which also confirms the tact at 171680 cycles).
One more thing that comes to my mind would be testing a demo that uses covox sound and that would be pretty much all.