Not sure where the VIN is on the engine, but even if you had it how would you be able to tell when it was really swapped and what the actual mileage is on it?

Really the only way to know is if they are truthful, and have paperwork or receipts from when they purchased the engine and had it installed (if they didn't do it themselves, which most people don't). So it's kind of a crapshoot on vehicles with a different engine in them.