Well, his Biography and his teaser story - Demacian Heart, describe his most outstanding magical trait as akin to truesight/magic detection, though it also states that he was unwittingly a decent conduit for magic.
From Sylas' Biography:
Noting the boy’s curious ability to sense magic, they used Sylas to identify other mages living among the citizenry.
...he accidentally brushed against her skin. The girl’s magic rushed through Sylas’s body—but rather than killing him, it shot forth from his hands in raw, uncontrolled bursts. It was a talent he did not know he possessed, and it resulted in the deaths of three people, including his mageseeker mentor.
Also worth noting is that, apparently, Sylas was particularly adept with magic, having taught Lux quite a bit during their contact in the prison.
Even with his shackles, Sylas recognized her as a singularly powerful mage, and over time the two forged an unusual and secretive bond. In exchange for Sylas’ knowledge of the control of magic...
So, between having magic detection, a natural affinity for magic, and the ability to serve as a conduit for the magic of others, he's got a fair repertoire of abilities at play.
On an unrelated note, for somebody who's supposedly all about mage freedom and rights and whatnot, his in-game kit doesn't exactly scream mage. All his abilities, aside from his ultimate, involve the control and usage of his chains. Is he keeping them as an accessory, or can he simply not take them off?
It could probably be that he can't take them off, but IMO, it's all the more likely that he keeps his shackles on as both a powerful weapon and a symbol for his rebellion. Since they're made of petricite, and he knows how to store and channel magics through them, he has a reliable and powerful weapon to use - everyone else's magic.
And, as a rebellion's symbol, you can't get much more inspiring than a man tearing down oppressors with the chains used to bind them.
