personally, if possible, get an exhaust gas test as this will give everything you need to know about what the engine is doing.
CO/HC/02/CO2 and Lamda...or as many of them as you can
test it from cold at idle and 3k revs...let the car idle for few mins before taking read, plus few mins at 3k
again at mid temp at idle and 3k
again at full temp at idle and 3k
make a note of all the results along with how quickly the reading rises/falls
this will give you a big heads up where to look if there is any issues, CO and HC give the biggest clues.
follow that with a full compression test as mick suggested, making a note of max compressions and how many engine strokes it takes to reach that point.