How to identify OEM vs aftermarket heater core?

If you have a air supply, even a bicycle pump, and a way to seal the ends to keep it under pressure (16 to 20 lbs) and a gauge on it over night. if it looses pressure bad, if not good. also submerge it in a tub of water look for bubbles. I like the hot water circulating through it some expansion could cause a leak BUT I suspect an overnight leakage test would show that up too. ( just be sure its not leaking at the ends where you have the plugs etc.)