So, some features of magic are really hard to fake. Mostly self-targeted things. The sensation of casting a spell is pretty much unfakeable, but spell effects outside of yourself can obviously be faked, and you can be contained so that once you've cast the spell the energy is broken up and dissipates.
I have a talent that I've picked up that... it's very rare. For all I know I'm the only person with it, I developed it myself and have tried to keep its existence quiet. When I - When I kill someone, I take a bit of the magic that they had. Works for people who don't do spellcasting, they still have energy built up.
Works for orcs.
Nerikross could have simulated it if he was clever about it and actually let my spells be cast in the real world and had lined up a bunch of people to die just to keep this believable. But he's not that ruthless and he would have had to know about it ahead of time which he didn't. Alfirin is that ruthless but I still don't think she knows about this talent and it's still quite a length to go to just to convince me this is real when she has better options for that and still no good reasons I can think of to bother.