I've recently bought JVC HR-S9600 VCR. It was bit of a eBay
risk, but on first glance it looks it was worth it.
Machine is inside and outside very clean. There are few scuffs and scratches from normal use. otherwise it looks like NOS. Came with original remote and manual.
I played few tapes recorded from analog TV back in the day and quality surprised me. I would say almost like DVD. For sure better then I remember them :-)
Dynamic drum looks like it works fine. There is short buzz on plug in and on power up. This is possibly check of the gears. On fast forward or rewind while playing, drum moves accordingly. Buzzing noise is there longer time which makes sense. Only it is quite audible, but I had machine open so while it is closed and in some kind of a hifi rack it could be acceptable even on new unit. Picture is all the time stable with only thin noise bars while rewinding or forwarding.
But... And there always is at leas one but. I did not tested it more while I had not much time last week.
So are there any methods except to look how nice the picture is? from test equipment I have 100kHz cheap chinese "true RMS" multimeters and two channel oscilloscope. I can borrow 30kHz true RMS Fluke from work if I need.
Also service manual might come handy just for case something (terrible) happens.