Your best bet is if the car you're looking at is stock or mostly stock but without knowing the previous owner it's hard to tell if it was once modified and returned to stock.
I compiled a checklist when I was shopping around for mine. Searching various forums, this is what I came up with. Hopefully it helps you.
During the test drive:
- smoke on start up (bad valve stems)
- smoke at idle after warm up (bad piston rings)
- smoke under acceleration/boost (bad turbo possibly?)
- rear quarter panel rust
- accidents? signs of paint respray, rough overspray in door jambs
- fender damage, messed up/missing fender liners from silly stance boy life
- bootleg wiring jobs anywhere around the car, check interior fuse box
- oil leaks
- worn serpentine belt
Signs of modifications and ms3 specific things:
- teeing vac lines for boost gauge
- possible signs that an access port was installed. usually signs of scratches by the obd port and access port mount taped/suction cupped, etc.
- possible signs of high pressure fuel pump internals installed. no real way to tell without access port data BUT, the install requires that you take apart a lot of stuff to get to it. missing/mismatch hardware is usually a sign.
- look for worn/mismatched bolts on dp and between the dp and cbe. Look for ripped insulation by the bolts, means work could have been done there. Look for worn bolts or missing bolts on heat shields.
- coolant reservoir is turning black/dirty due to bad head gasket. you cant tell a bad head gasket other than a few misfires on cold starts but the evidence is in the coolant tank where carbon has contaminated the coolant system and dirtied the coolant tank. people should look at coolant reservoir for obvious screaming issues
- bpv recirc hose clamp shows signs of being removed a bunch of times. could mean tinkered with a lot. basic maintenance requires removal too but should appear overly abused
- bad vvt (mainly affects first generation ms3's from what i understand). on start up, it will rattle for 1-5 seconds. can mechanically check it by taking off oil cap:
Since you have a mechanic, they should have you covered on possible mechanical issues.
Best of luck and keep us posted!