Some here have much more knowledge than I do. Some good advice by others.
Three simple things that I do when purchasing any used car or truck.
1. Check the engine oil.
2. Check the transmission fluid.
3. Check under vehicle for leaks.
I'm usually purchase from private sales. But this applies to used dealers also.
I am amazed at how stupid some sellers can be.
I have viewed used cars/trucks and in checking the oil it is either dirty or down. If it is dirty - then how good did they take care of vehicle. If oil is down. Then only (mostly) two things occured. Either it leaks oil or it burns oil. If the vehicle is a beater and low cost maybe not a big deal. But if paying a few/many grand - then move onto another vehicle.
I am amazed at viewing cars/trucks that have burnt/brown/dark gray automatic transmission fluid. Mostly, it should be pink in color.
If one is selling a car whether a private person or used car dealer one would think that for a few dollars - that they would put fresh oil in car and fresh automatic transmission fluid in car.
And I love the ones that state something similiar to: Our cars go through a 41 point inspection before it goes on the lot.
Only to find what I have stated above.