So I didnt see anything mentioned in the rule book, but we have always played it that is someone used a psychic power, it was apparent that they were the ones using it (ie there was a glow in their eye, their hair stood up, etc). As a character that was until recently unknown to have psyhic powers ( i didnt use abilities and said i was a scum) i was wondering if this is indeed the case, can you immediately tell who a psyker is when they use any ability (not just the obvious ones like Firebolt etc)

There isn't any visible effect unless the power has one, or you roll on on the tables.

My group has had this come up before too. I think I really should just be how your group think that powers should be portrayed, even in some of the fluff if a lot of psychic powers are used there develops a layer of frost(which could be a phenomena roll). It would seem pretty fair that if you are using all you power dice that you look like you are concentrating hard(ex. furrowed brow, sweat on forehead, bulging veins etc) to if you are just using a fraction of your power dice it would be harder to notice. But as Dalnor said there are now RAW visible effects unless they roll on the phenomena table. I would just sit down as a group figure it out and then run with it

This could also depend heavily on the discipline of the power being manifested. In the description of Telekinetics, for example, they actually mention that nosebleeds are not uncommon for a Telekine practicing his craft.

"Hmmm. Raphael just got smashed to the ground by an invisible force... and that guy over there just had an inexplicable nosebleed."

"What does that mean?"

"I dunno. Let's kill the nosebleed guy and see if the invisible force comes back."

Hmmm. Remind me to have the scum in our group get "nosebleed on command" implants for distraction purposes. *evil grin*