It would mostly depend on what skill was being used. Negotiation goes without saying, that's two people haggling over price and if the NPC gets the better of the PC, then that's how it goes.
With Coercion I would typically (assuming the NPC succeeded) apply some strain and tell the player, "you feel this person is definitely someone you don't want to mess with". If he still wanted to start something, I'd maybe assign him a setback die or two to illustrate that he's scared of the NPC, or maybe make him roll a Fear check - whatever seemed most appropriate.
For Deception checks I try to roleplay the results. If an NPC makes his Deception check I speak his lines in a straightforward, reasonable tone; if he failed it, I throw in lots of hesitation, flickering eyes, nervous gestures and the like. I never tell my players they have to believe or disbelieve.
Charm is a little trickier. Just a couple of weeks ago I had my group's bounty hunter do a personal job for his guild handler, one he wasn't supposed to tell anyone else in the guild about. A fellow NPC hunter wanted to know what all the fuss was about and made a Charm check. She was a Falleen with the Charmer specialization, so she crushed his Cool pretty effectively (several successes and a Triumph). I told the player "she seems very trustworthy and helpful", to which he replied, "Do I have to tell her what's going on?". I answered, "you don't have to tell her anything, but you think she's on the level". He wound up giving her some insight into the situation and had her tag along for the investigation.
So the short version being, let the skill check be a guideline for how to convey the situation to your players, but let them make the actual decision themselves.