canary check
Taken from the times miners had a Canary down in the mine shafts to detect deadly gasses, this is a check of a toilet after destroying it with your bodily waste to make sure it's clear.
"I'm just gonna use the loo."
"Hang on. I was in there. Let me give it a Canary Check."
"Hang on. I was in there. Let me give it a Canary Check."