If you seen someone printing and it was very obvious to you, would you inform that person or let them go?
I'd let it go. You never know what their reaction would be. That's just my opinion, not saying it's right.
I'd would watch them and see if they look like they would take it the correct way. If they looked and an A-hole, i'd let it go.
I think I'd agree with observing them and then deciding. If also probably try and make it quick and just say hey your printing. Most likely... I'd just take note and carry on. I guess it depends on how bad it's showing and where I'm at...