It’s been years since I saw it - and I’m sure I wasn’t looking at that detail. If I were doing the sketch, I would buy lunch for the cast in rehearsal and keep an extra couple cheeseburgers, let them cool down, and use those. If that is the case, they were using previously cooked burgers. Why wouldn’t that look real? A couple frozen patties could be used to add verisimilitude. Both could be easily switched out.