The is no tag on the engine that will correspond with the car VIN.
There are tags on the back of the block, right below the 2 half moons on the head, but they are not in any way tagged to the cars VIN. So basically there is absolutely no way to tell if the engines been replaced.
Unless the guy has paperwork showing prove that he did what he did or if the engine is a different engine (so not an M52tub25), you should not believe him.
But that doesnt matter, why are you scared of the miles. Miles mean nothing on a M52tub/m54 motors, its the way they been take car of. I've seen some M54s that been toast by 150k and many that run better then new even after 400k.
Get maintenance records , prove that the cars been maintained, using correct oil etc ...
good luck , and I hope you arent paying more then 1000 for that car . Are you?