Best thing about card magic is that it has so many possibilities, that you can always improvize. So if a spectator wants to shuffle the deck, let him do it and show an impromptu trick. Anyway ... I have practiced shuffles for a long time, so I can always do false overhand shuffle, false riffle shuffle, Rosetta shuffle, circulation shuffle or other false shuffles to convince spectators that the deck is really well shuffled, although the truth is different ... and mostly they "buy" it ...

I recommend Lennart Green's Green Magic volume 4 and other Lennart Green's videos - there are many convincing false shuffles, and they look very natural after some practice.