I've had this same thought.
Something I've been trying lately as GM in Dresden and other games is to be pretty up-front with the players when someone is lying. Even going so far as to say, "yeah, this guy is totally lying to you... :wink, wink:" I tell them they don't know what exactly he's lying about, or why, or what the truth is (nor do they have enough information to call them on it). Then I let them roll Empathy. If they fail, their characters don't know he's lyin (or aren't sure), but the players do. My group has actually had a lot of fun with this. I think that the FATE system actually does really well with this separation of player knowledge vs. character knowledge (for the same reason, I tend to be pretty out in the open about aspects to the players, unless there's a specific reason to keep it hidden). I think the tension of knowing they're lying but also knowing you failed the roll so your character doesn't know is more tense and more satisfying than the players not knowing they're lying.
This technique is actually from another game (Dogs in the Vineyard by Vincent Baker), but it's worked very well for my group. I also think it fits well with the advice in DFRPG about only rolling the dice "when there is an interesting challenge with meaningful consequences" (YS 192).
So looking at it that way, if it isn't an interesting challenge with meaningful consequences, just tell them "no, he's telling the truth" or "she doesn't know anything" or whatever without rolling.
Basically, I'd assume they're always trying to tell if people are lying and I'd only draw attention to it (and bring out the dice) when either the target is lying or else there's some other reason that rolling the dice would be interesting and meaningful. And then, like I said above, I'd still start by saying to the player, "she's totally lying to you, dude..."