The Whiskey Taster?
The man takes a sip and says immediately, "Excuse me, I think you may have made a mistake my good man, this is 10 year old whisky."
The barman apologises and thinks, well, okay maybe he can tell a 10 year old be what possibly the difference be between a 20 and 40 year old, so he pours a 20 year old.
"This is no good!" exclaims the man. "This is a 20 year old whisky! I can tell the age of any whisky by simply tasting it."
By now interest has developed in the situation and everyone in the bar urges the barman to test him out, all saying they'll pay for each shot.
After about ten whiskies, all correctly guessed to the admiration of all, the man says that he has finished for the evening. An old guy emerges with a wee goldie.
"If you could indulge me, son" the old man says, "could you try just one more?"
The man shrugs, takes the proferred glass and downs it.
Instantly he splutters and yells "That tastes like p*ss!"
"Good guess!" the old man replies, "Now tell me my age!"