Time to look under the hood. Open the hood. Look at the condition of the engine. If an engine has been worked on, you will see newer and older parts. If the engine is not original you might see a chalk marks from a junkyard or a fresh coat of paint. Inquire about any suspicious parts. Look at all the belts. Make sure they are not cracking or tearing.
Check the hoses. If the engine is very dirty or you see spill marks oil or antifreeze caked on the radiator, this is a bad sign. Are the clamps in place or can you see marks on the hoses? Look at all the electrical connections. Are they like new or have they been worked on. Check for loose electrical tape and wires that are not connected. Check for excess gasket maker (looks like a blue calk around the valve covers) or any badly done repairs.
Now let’s check the oil. Make sure you have a rag or paper napkin. Make sure the car is level and the engine has been stopped for a while. Find the dipstick and pull it out. Clean the oil with your rag and place the dipstick back in. Pull it out again and look at the oil level. At the end of the dipstick there should be a flat surface sometimes with a crisscross pattern and an empty and full etching. The oil should be wet the stick above the middle mark of those etchings. Also look at the color of the oil. If the oil is black and runny then it hasn’t been changed in a while. The engine oil should be a dark honey color. If the color is a milk color then the engine may have serious problems, stay away. Place the dipstick back and remove the oil filler cap (with the engine OFF!). Look at the inside of the cap. If you see water or a milky oil then stay away. Look inside the oil filler hole. Look for any buildup or any damaged parts.
Check the cooling system. The water in the reservoir should be filled (between max and min marks) and the liquid should be clean and transparent (coolant may give it color). The engine should be dry. Inspect the radiator. Look for cracks or repairs. Look at the fan and fan clutch and check for any damage that could have been caused by a flying part.
Check the battery. Are the battery terminals clean? Look for scratch marks as a sign the car has been jump excessively. Although corrosion on terminals is common and does not mean the car is not good, it can contribute to your overall impression of how well the car was cared for.
If you have the time, tools, and car owner’s approval, you can check the spark plugs. The condition of the spark plugs can tell you much about how well tuned the engine is. I will not cover spark plugs but you may want to read more about it or ask a mechanic.
Find the brake fluid reservoir and see if it is filled to the proper level and if the fluid looks clean. Is any fluid leaking? Are all the hoses intact?
While you’re at it, check the air filter. This should be easy on most cars. There may be some retainer clips or wing nuts to open. More importantly, check how clean the air ducts are. If the air filter is filthy, the owner has probably overlooked more things, beware.