Blowing the bullshit whistle is method of calling someone out for not telling the truth about something. When a person knows a statement or story is total bullshit, he will make a “tweet-tweet” sound like a steam whistle. It’s especially handy in group settings like a smoking circle where the embarrassment for the liar can be maximized.
Todd: “Yo’, Chris. I heard you and Rabbit went to the club last night. How was it?”
Chris: “Dude, it was crazy. The bitches wouldn’t leave me alone. I guess they all wanted my potato dick.”
Rabbit: (exhaling an especially big bong rip) “Tweet-tweet. I'm totally blowing the bullshit whistle on that.”