Leo tells Phoebe that they were never supposed to find out he was a whitelighter, yet in all future episodes it is established that witches know their whitelighters.
When Leo uses his power to refill Phoebe's glass at Quake, the ice in the glass does not float to the top. The entire time they are sitting there, the ice never floats to the top. Even after she lifts the glass and drinks from it. It's likely that the "ice" is fake and is hiding something used for the trick.
Further evidence of the "trick" used in the water glass/ice-cube debacle at Quake with Leo and Phoebe: the water swirls and jets up from the bottom of the glass, creating the only movement of an 'ice-cube', when the water pressure from below is too strong and one of the glass 'cubes' is forced off the pile.