We’ve all done it. Someone that we are talking too has a snot-encrusted wad of mayonnaise on their left cheek.
We politely point to it and wipe off our own left cheek as a gesture that they should wipe it off themselves. Invariably, they wipe off their right cheek first and we have to correct them.
Why is this? Is it confirmation bias or something in the way we relate/look at each other?
My SWAG - mirror response
My husband actually does that also - if he gestures to his right cheek, he’s meaning for me to check my right cheek.
I do the opposite - if I’m looking at him and he’s got something on his right cheek (my left side) then I’m going to point at my left cheek to tell him it’s there.
He says that’s because I have bad spatial skills, but I say it’s because I’m more empathetic (mirror neurons) than he is. 
Usually it’s something around the mouth so I signify with a downward wipe with my thumb on one side and my index finger on the other whether I’m the indicator or the messy person and most of the time, I remove the offending particle.
So surely there are two people who do this the “correct” way. People I tell never wipe off the correct side, and when people tell me, I never wipe off the correct side. Perhaps it is confirmation bias, but I can’t remember the last time, if ever, someone got the correct side of the face. 
Use your words. I just tell the person which cheek to wipe.
Just wipe it off for them. Not your fault people can’t tell left from right.
Preferably with a big tissue you just spat into
Look at the spot while gesturing.
If people invariably choose the ‘wrong’ cheek, that would suggest that you are the one who’s mistaken, rather that ‘everyone else’.
If I wanted to silently alert someone that they had something on their *left *cheek, I would point/wipe/whatever my *right *cheek, while looking at the correct spot on their face. They will be able to tell where my eyes are looking, and that ought to be enough to clue them in.
I’m left handed. When I was in little league, I had a coach who expressed some concern that it was hard to teach me to swing the bat, because he would have to switch hands and swing wrong- (left-) handed. I told him, “Uh, just bat normally in front of me, and I’ll just mirror your actions.” The concept utterly confused him.
When I mime to someone to wipe off their mouth, I do so as if I were a mirror image. In my experience, the vast majority of the population correctly takes the hint. My wife, on the other hand, does not - she does it the way the OP does.
Like “No, your* other *right cheek”?
I read somewhere, and I can’t remember where, that it’s a gender related thing.
When a person has something on their left cheek, a woman will point on their own right cheek to tell them, while a man will point to their own left cheek.
So basically it gets screwy when the two people are of other genders.