Foursquare is like a community of insiders. They will how to get free Wi-Fi at local spots, and they will also tip you on the things you can order off the menu at restaurants and bars.
A quick search for "off the menu" brought up some treats matching the phrases "secret menu", "not on the menu", and "off menu": a drink called a William Blake, truffle burrata, cauliflower appetiser... and more foods Foursquare users said were the best at those establishments.
Like items on fast food chains' secret menus, this could be hit or miss, depending on your server and when you go, but it doesn't hurt to ask.
USA Today has a list of secret menu items at 10 restaurants around the country.
10 super-secret menu items shared on Foursquare [USA Today]