anyone know how you identify OEM Rockshox Pikes without taking them to bits?
LBS has a set of 454 dual air Pikes (no u-turn) for a pretty reasonable price
they have a cut steerer and look like they've been attached to a bike so I'm thinking they've been taken off a stock bike – the guy in the shop didn't seem to know
how can I tell if they're OEM and will there be any issues with them if they are?

I would say that its a fair chance that they've come off another bike (which is probably why they are going cheap…).

Which would probably make em OEM.

Warranty would presumably be 12 months only. The shop should know though.

Which is why shops are reluctant to swap stuff off stock bikes as it creates problems for them selling the stuff on.