There are a couple of schools of thought on the R500 engine...
-fragile little barstewards, don't touch
-I've used and abused mine for a million track days and still going strong.
I'm a bit in the first camp.
In 2001 (I think) there was the R500 engines go bang saga started by Richard Ince at the ring and with his race cars. Caterham eventually brought out the package of fixes. VDV (?) uprated mains, diddy apollo tank, ecu controlled fan, oil cooler. I think of those the main saviour was the new bearings.
So if buying, you need to check that it is not pre-saga engine. That's pretty unlikely.
Then you should check for a sensible and documented history of use versus refreshes.
In terms of checking the health of a current engine, there is little that can be done that doesn't involve a strip, measure, rebuild. You can do a compression test and leakdown test and I don't think much more.
So if it has low use post any previous rebuild, has the mods, has good compression and leakdown, you should be ok. Well as ok as you can be with an expensive engine made of cheese 😬
(2 R500 blow-ups) Graham