Engine swap cars can be a hard buy. Usually if someone did the swap themselves they know all about it. They know exactly what that "Noise" is, they know when/how to beat on it, and also when to take it easy.

A lot of swaps are common now aday but you just have to be careful. Drive the car. Wait a week and then come back and drive it again. Take it to a mechanic and get them to look at it.

Look for things that aren't suppose to be in the engine bay, see how clean it is. I've seen many motor swaps with bottle caps blocking off valves, zip ties everywhere, PVC intake/intercooler pipes. I've seen some really junky stuff. But I have also seen really nice swaped and great deal cars.

Just don't jump on one the instance you see it. Drive, drive, take it to a mechanic, listen for noises, check for leaks/smells and you should be fine. If you feel like there's something wrong then just don't get the car.