For the RC airplane to have a sudden uncommanded roll - there can't be too much other than something that has caused one wing to have more lift than the other. The wing section stalling at the inboard location will make a pretty small rolling moment compared to the outer wing section so a rapid roll is probably tip related.

If we look at the things that will cause a tip stall in our models (and they are noted in detail in the many posts above) they will all be related to flight conditions such as tight turning, high wing loading and control deflections - all initiate or add to a tip stall condition.

Then if you see a heavy scale ship on a too slow landing approach suddenly drop one wing and fall out of the sky - there is a tendency to call it correctly a tip stalling condition. You might not know for sure since you can't observe the flow characteristics of the wing but based on the airplane and it's flight characteristics it can be pretty correctly called by an observer.

If it isn't a tip stall but instead is a full wing stall the airplane won't do the roll motion, such as Dick's small foam airplanes in a full stall mode. If the airplane is going straight and pulled up into a stall on purpose in general the stall be straight ahead, very few airplanes will seriously drop a tip unless the high turning, etc. conditions are met.