she's better than your average PSI-cop. She doesn't even need to be in a clear line of sight, as Nemesis suggests.
I'm sure she can since she was reading the minds of aliens during a combat situation many times throughout the series. It's interesting to note that she was originally conceived as telepathic like her mom and later downplayed as empathic instead since she would give away most of the plots before the real danger could build up. The original commercial teaser for TNG (shown at one point at the beginning of 1 of the ST:IV releases) described her as "the telepathic Troi." But Troi was written out of several 1st season episodes (e.g. "Hide & Q,") because tyhe writers couldn't figure out a way to avoid the question of why Troi wouldn't have sensed the deception if she were present so that always had her "away" at a conference or visting home at the time. Sirtis said that she was almost written out of the series at one point after the 1st season.