internal parts through the oil filler neck

If it's possible, look at the internal parts through the oil filler neck (Don't open the oil cap while the engine is running!)

Dirty engine Clean engine

Oil sludge inside the engine

With the engine turned off, try to open the engine oil cap and look inside through the oil filler hole. Use a small flashlight. If it's possible to see any internal parts there, check their condition. What you are looking for is the oil sludge - this thick black buildup on internal parts you can see on the lower photo (this is how the sludged up engine looks inside, with the valve cover removed). If you see a lot of black oil sludge buildup inside the engine it means that either the engine oil hasn't been changed for a long time, or the engine has been overheated, or poor quality oil was used.
In either case, avoid buying such a car - sooner or later it will have serious engine problems.
Well maintained engine usually looks pretty clean inside; if you check few cars of the same model it's easy to see the difference. However, again, if the engine looks clean inside it doesn't mean that it has no other possible problems - as a last step, have the vehicle inspected by a mechanic of your choice.