New classes, new rules and such are not, in my opinion,the answer to this conundrum, and rarely are they the best answer to anything.
Simple role-playing and DM adjudication based on common sense, however, are.
Let's say your would-be con man is intent on deceiving the local constabulary into believing that, yes, indeed, this is his very own diamond-studded donkey (or whatever). If the player is of a dramatic bent then you, as DM, can say, "Okay, act it out. Deceive him! What do you tell him?" If, on the other hand, the player isn't an actor-type, then simply have him or her explain the nature of the lie to whatever level of detail. If the character's Intelligence score seems high enough that the character might convincingly reason out the logic of the lie, and if his/her Charisma score seems high enough to "sell" however outrageous the lie, then the PC gets away with it. If not, then no.
Alternately, you can just judge the odds that the deceived will buy it and roll some dice.
AD&D is most certainly the right type of game in which to do this, because it never has the gall to tell you that you can't.
If you want to be a con man, start lying.