when cold they tend to be relatively quiet so get the guy to take you for a 15 minute drive in it and ask him to floor it aswell, look out the rear window for smoke when hot
once shes back lift the bonnet and rev the car between 2-3k rpm and listen out for bigend knocks (dull heavy thud type knock) if you can hear that then walk away unless hes gonna drop massively on price allowing for a new lump.
check for excess crankcase pressure
slipping clutches
noisy clunky whiney gearboxes
leaky headgaskets
plus give the bodywork a good checkover (especially underside) a couple of little rust patches on rear arches is way better than buying one that looks mint on top but a rusty jacked up mess on underside
just bare in mind your not gonna get a mint faultless car, virtually all of them will need some degree of work but you need to separate the lemons from the shiny apples with a maggot hole if you know what I mean.
one more tip.....if car looks immaculate and been blown over at some stage bring a magnet with you as it may well have just been tarted up to sell.
also if car comes with loads of history take this with a pinch of salt as many of them parts previously fitted may have been removed (including forge engines)
don't pay any attention to mileage as it means nothing on a 20 odd year old car unless its backed up with full history from japan from when car was new, too many have clocks changed at some point or have been abused on track etc.