Alternatively, "they're hiding something" could simply be a case of general observation and perception, rather than Deanna using her empathic abilities. She might not be able to read the Ferengi in the conventional sense, so she's switched over to using good old-fashioned body language indicators instead.
So.... the only person on the bridge qualified to read body language is Troi? Troi stating the obvious is one thing, but to be the only one allowed to do so just speaks volumes at how incompetent the Federation is. No wonder these bozos would be outmatched by a couple dozen ferengi later in the series.