1) If the owner has already started the car and had it running before you even arrive to see it (assuming you’re meeting at their place).

A cold start will reveal issues like:

-dead battery (which can point to poor maintenance or neglect)
-a failing starter
-bad idle or smoky startups (not talking just water vapour)
-smells inside the cabin (if the owner has been running the HVAC to blow it out)

2) Under the hood, the engine bay is dirty, not from dust and debris like leaves, but from fluid splatters (i.e. oil or power steering fluid)