The mechanism varies with the warhead. The PG-7 uses the acceleration from launch to set off a primer. This starts a slow fuse that will cause self-destruct if the grenade doesn't hit a target within a few seconds (so you don't have a live warhead cluttering up the place, requiring an EOD team to dispose of it.) It also burns away a safety plunger that allows the piezo-impact fuze to complete it's circuit and ignite the shaped-charge.

So to get one to detonate manually, you're going to have to simulate the 40-ish Gs of a launch, the delay of flight to allow the safety to disengage (but not so long that the self-destruct happens), and be far enough away to not get injured by the explosion. That's a challenge.