(IE "Oh, yeah, I know, the bartender actually owes the thieves guild 500gp, so he's going to look the other way while their guy slips poison into the character's ale").
That's what random determination is for. This seems to be a lost art!
Look, you don't just fiat the bartender's behaviour. If the bartender is not a developed NPC, you might roll for his alignment, you definitely decide what the chances are that a typical bartender in his position could be bribed/bluffed/intimidated by the Guild, then you roll to see. If in doubt, default to 3 in 6. You DON'T create a malevolent universe where everything is out to get the PCs. In the bartender case, if the PCs have treated him well there's probably a chance he'll go straight to the PCs and inform on the Guild. You roll for that, too. I've seen cunning NPC plans unravelled by unlucky rolls, just as much as with PCs.