Used as a connotation for telling the truth.
Relates to a question of whether a sandwich had be in the fridge all day and whether the answer was truthful.
Can then be used to test the honesty of any answer.
This colloquial phrase is commonly heard in modern millennial social circles and among co-workers in fields of work involving the skilled trades, etc. Used primarily as a call-and-response, but can also be used formally in the pejorative sense.