Well, do what I do in my campaign. My PC's are just like their characters in that they never know when an NPC is lying to them, but they can suspect. What I have them do is if they (out of character)suspect that an NPC is lying to them, I have them roll an imaginary Sense Motive check and then I roll as well to keep them guessing. As for Diplomacy/Intimidate, a strict reading of the DMG says that PC's have free will and under no conditions can they have their attitudes adjusted w/o magical aid. So if a key NPC wants to "recruit" a PC, he/she invites a good candidate (usually someone w/ a low WILL save) and then casts a heightened Charm Person spell.
As for the PC using his skill as a person and not a player, I award a bonus (ranging from +1 to +5) for a good point. A player must still make a check (making it fair for those who actually pour skill points into these skills) and be successful. That way a bard will always be decently playable choice.