You know that GradeAUnderA vid where he said to use proof to prove a catfish to be true or not? Well this is the same thing but with art. This is a sure fire way to catch them
Just say "Show me one of the layers then"
They have NO EVIDENCE of it, they will likely say "I drew it on one layer"
Which you would respond "WHAT ABOUT THIS?!" And you would show a pic of your layers while drawing the piece like this prnt.sc/e4fad4
This method only works if you have more than one layer and that your background is hard to replicate