I look for a clean engine - not just one that has been recently detailed but look for signs of oil/coolent leaks.
Look at the condition of the air filter as well as the smell and color of the trans and oil fluids.
If you can look between the condenser and radiator check for debris (leaves, dust and dirt).
Check the condition of the transmission and oil dipsticks. If the dipsticks show any sign of burnt spots then there is a good chance that the engine has overheated (run don't walk away).
Listen to the engine running - observe any unusual sounds. Look for any leaks expecially around all gasketed areas, seals and water pump.
Check fluid levels and tire air pressure.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles