when in doubt, whip it out
"It" is referring to showing your male genitalia to a certain lady (or guy if that's what your into).

This is the common reply you give to your friend when he continues to complain whether a certain girl is into him or not.

It's the ultimate test. If the girl doesn't take bait or if she's about to charge you for harassment, proceed to walk out the door and move on.
Pete: Hey Billy, Sara gave me a wink yesterday, but sometimes she doesn't seem interested in me.
Billy: Bro, when in doubt, whip it out!