Final answer:
In the restaurant industry, a clean and folded napkin or a table tent with the word 'SERVED' or 'CHECK' is often placed in front of customers to inform the managers that the server has been by the table.
Step-by-step explanation:
To inform the managers that the server has been by the table, a common practice in the restaurant industry is using a signal such as placing a clean and folded napkin or a small table tent with the word 'SERVED' or 'CHECK' written on it in front of the customers. This visually communicates to the managers that the table has already been attended to by the server.