Most of the fast food in the areas I've traveled to have switched to unscanable systems. I'm sure some are still out there that can be scanned but the easiest way to do d them is probably going to be using something like closecall or signal stalker.
The ones around here are BlueTooth, especially the In-N-Outs, who (from time to time) put one of their employees to walk the drive-thru and personally take orders, especially with the line(s) is(are) long.