The staff of the Sonic I went to were very nice and informative. I ask about "dedicated" fryers. Although they had 3 fryers, I was told that their Breaded Cheese Sticks were also fried in one dedicated to only potatoes. So, you just have to be bold and ask and if still in doubt, don't get it. I don't think a lot of fast food places are aware of cross contamination...in fact, that is a problem even in restaurants who claim things are gluten-free.
I tried to link to another post on this topic but it didn't work. This is taken from the Alamo Celiac support group's website:
Sonic. 866-657-6642. No gluten-free menu that is printed or published on the Internet. However, a company representative at Sonic's toll-free number verbally gave the following list of gluten-free menu items:
Breakfast sausage patty
Hamburger, no bun
Hotdog, no bun
Sliced American cheese
Shredded cheddar cheese
Egg patty (may be contaminated from cooking on the same griddle as non-gluten-free foods)
Salad dressings, all varieties in the usual menu*
Shake mix (Shakes may be made in the same container as malts. Ask the staff to clean the container to prevent cross-contamination.)
Softserve ice cream
Hershey's chocolate syrup
Butterfinger, M&M, Reese's Pieces, and whipped toppings
Route 44 drinks, all flavors in the usual menu*
Slushes, all flavors in the usual menu*
*Special menu items are not included in this list.
Although Sonic's Tater Tots and French fries are gluten free, they may have gluten contamination from being fried with gluten-containing menu items.