The tampered pager likely looks nearly identical, and even un-tampered pagers will vary a little bit from manufacturing. It's possible an expert might be able to visually distinguish that a particular strand of wire is the wrong gauge or the soldering pattern suggests it wasn't made on the appropriate machine, but there shouldn't be something obvious.

presumably the only difference between an explosive-laden battery and a normal battery is it's capacity. All else will appear identical. And tearing down the battery to inspect it destroys the battery.

Well usually what you do here is start randomly sampling.

Lot's of pager batteries are the shrink-wrapped cylinders with 2+ cells, so I'm guessing it might be possible to dress up one of those cells as a dummy w/ explosives instead.