From my experience, you can shieldpoke almost all the time if you get the last bottom hitbox of nair to hit. The final hit of nair is basically this "pulse" of electricity that is notably bigger than the multihit ones. Do nair while hovering upwards slightly and it should poke. The reaso why i say bottom is that its hard to get the top to hit, since it requires you to basically be under them, which is not possible unless they're on a platform and you're hover nairing under them.
If they angle their shield up, you should be able to up air them and it should hit. I can't confirm this, but it should work. Or you could do a turnaround TP into hover nair and then rise upwards as you nair, and then after that, back air. The tail goes from down to up so it can creep under the shield and hit 'em.