They must know the difference between a fart and taking a dump.

When I'm home with my dogs and they need to shit, they'll come to me and let me know that they want to go outside.   But if they have gas, they'll just lay there and fart away!  

They must realize..."hey, I feel a fart coming...I'll just lay here and let 'er rip....it is only gas"   Or  "oh...gotta pinch a loaf..where's my owner..need to go outside".

On the other hand, could it be a physiological response to intestinal pressure?

Like X amount of pressure or < = fart
Pressure > than X = Shit

my will watch what im doing, and can tell when something bad is about to happen.

What I want to know is how do dogs pinch off a loaf so clean that they don't have to wipe their butts?