A light-blue hanky in your left back pocket means you want a blow job. Right pocket means you give one.
7 seconds sound clip from the Cruising movie soundboard.
You can hear this line at 00:26:17 in the Blu-ray version of the movie.
Quote context
[...]
- The victim was a teacher at Columbia. They found him in his apartment in about ten pieces.
- Detectives came around asking questions for a couple of days, nothing came of it.
- I guess you can't be too careful, huh?
- Excuse me. Could I ask you about these?
- The green one left side says you're a hustler, right side you're a buyer.
- The yellow one left side means you give a golden shower, right side you receive. The red one is...
- I'm gonna go home and think about it.
- I'm sure you'll make the right choice.
- No, I just... I like to watch.
- Later.
- Yeah.
- If you like to watch, take that hanky out of your pocket, asshole.
[...]