We've been taught philosophically there is a "hard plane" on onside kicks.
I have been taught this as well. I've also been taught to treat every kick as if it were an onside kick, until it is kicked deep. With that in mind, I try to follow this progression:
1st) did he break the "hard plane"? if no, play on
if yes, 2nd) was it kicked deep? if no, offside.
if yes, 3) did he cross the restraining line by a full step before kicked? if no, play on.
if yes, offside
it looks alot prettier with a decision tree in excel...but hey...thats the nerd in me.
With the benefit of Sky's screenshot, I would say this probably should have been called offside....that said...the guy freakin kicked it with the backside of his foot while doing a crazy riverdance move, and STILL delivered a perfect onside kick 11 yards deep.