The quick ducking action taken in a puclic restroom to see if any closed stalls are occupied. Often performed to assure safeness in performing a private conversation, whistling, or loud, obnoxious singing. If a duck check proves the company of another restroom-user, silence will ensue.
Bob: Yeah, so then she told me...
Rob: Hold on let me do a duck check.
Rob: You're good. There's no one here.