You can only tell from the games you watch, and it probably helps to watch a game where you don't care which team wins. I have plenty of examples from when my team plays, but when you see them making the same bad calls against other teams, you realize that it's consistently bad across the board. It's not even a matter of how many bad calls there are, but when they are made, since a string of calls can deprive a team of the opportunity to score.
I'm watching the NO - Denver series and I see Sean Marks set up outside the semicircle perfectly motionless and he gets called for a block as some Nugget runs right into him. I see Byron Scott pick up a technical, protesting a flagrant call on his player, who merely grabbed the arm of another, who managed to loose his balance and fall to the ground.
All this is happening in NO, as they are loosing their lead. It's like the refs are doing everything they can to help the Nuggets win.
I don't care who wins, but I do want to see a fair game, and bad calls mess one up.