They proudly advertise ribs that "fall off the bone"
If eating the barbecue doesn't require your central and lateral incisors (and the meat could be sucked, with just a little effort, through a straw), the restaurant is boiling and/or steaming the meat first and grilling or broiling it to finish, which gives brisket, ribs, and pulled pork the appearance of being cooked over flames. Properly cooked ribs may pull or tear clean off the bone, but they should never slide off. Amateurs might call it tender, but barbecue traditionalists know a fake by the telltale mushy, flavorless, and overcooked meat Jell-O they serve.
The menu or sign is all kitsch, misspelled words, and backward letters
If you pull into the parking lot of Bubba's Shak or The Jernt or Big Daddyz BBQ Hut, case the place for all of the above and proceed cautiously until you can confirm the existence of an active smoker. Generally, restaurants that play up the "we're authentic barbecue hillbillies who can't be bothered with spell check" schtick are banking on customers who eat up Southern stereotypes as greedily and carelessly as they scarf down boiled-and-grilled bar-b-q'd ribz.