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?
I'm not saying it's a "qualification". It's just the best thing she can bring to the table at that time. The others are probably inwardly rolling their eyes, thinking to themselves "We can see that for ourselves already, you stupid bint".
I mean, they could just ask Wesley what he thinks. I'm sure he'd
be able to tell them the Ferengi "are hiding something" too. Then Picard would tell him to shut up.