Sydney

Secretary thiefling mean girl in the Tower Arcane in luskan.
Now operates the night club in bonecity

Tower Arcane Initiation

Show them how to do basic spells and teach them how to do damage in a controlled test chamber. Meet a HR person. Sheirah / Sydney. Annoying mean girls voice. Purple shirt, well cut, uniformed skirt. Purple curly hair. Infernal heritage.

"Ucghs.. More wannabees." (Perc10) Quick face change to a fake smile. "Hiii. Welcome to the mages guild brave adventurers. I'm Sydney, here to assess you before sending you up to the elders."

"We can't take you to Arklem, he's very busy. We gotta assess you first. If you do well you can maybe see Celerum."

"Look they can give proper info, I don't know why all the soldiers up there revere Arklem so much; Celerum is way more handsome. But seriously though, we just got to assess you first."

Assess strengths. Let's try a cantrip shall we? (Expects basic sorc)
Now let's show you what real strength is. She'll cast fireball into a haybale room.

"Ray of Enfeeblement" on the constructs. They look like their posture shrinks (lack of strength).
Keep using http://dnd5e.wikidot.com/spells and expect them to up the ante or keep up with the mean girl charade.

Magically fortified to reset on request (old brotherhood magic, mordenkainen's fortress?)
If they do well, they'll get a pretty high rank immediately.
(Terms and conditions, do not betray the brotherhood or you will lose your rank, all benefits, weave recharge booths, and wizard benefits cards). [valid in Gael's emporium + brothehood]

Ok, so you are powerful. (Gain faction rank). That (flips through rulebook frantically) still does not prove loyalty.
Send them on a quest down to exterminate the gnoll caverns. Bring us proof of their demise. Take out their leader and they should scatter.