One trick that I use when looking at a car is also look around the yard. If the house/yard is generally maintained, it's a good sign. The other cars there can be used to give an indication as to the way other cars are treated.
Also, if you can, sneak a peak at the drive where the car was generally parked. Usually, when people are selling a car, they already have a replacement and people being creatures of habit, the replacement will be parked in the old car's usual spot. Have a peek to see if there is oil/fluid staining on the ground.
If, on phoning and after the usual "I'm enquiring about the car for sale", the seller wants to meet in a supermarket car park or petrol station, have some reason to go back to the seller's residence (examine the car in privacy is a good 'un). A genuine seller will not have a problem. A dodgy operator will not like it.....
As for looking at the actual machine, it's the usual points that others have mentioned. One trick that I use on the test drive is to ask the owner to drive the car back. Then you'll see how the owner drove the car and how gentle s/he was.....
Its an inanimate object you're buying not a new partner for life!
don't take it so seriously or you're definitely going to become a TIMEWASTER