A reality cheque can be an imaginary or crudely drawn cheque sized piece of paper you hand someone when it's clear their getting too detatched from reality. Implying that they should cash it and come back down to earth from their emotional or psychological trip in the clouds. Used as a funnier alternative to yelling "Reality Check" and can sometimes be more effective due to subtlety.
"Aww man my girlfriend didn't text back right away, she must hate me. It's all over i may aswell just end it."
"Dude, chill, it's probably fine."
"Aww it hurts bro, she doesn't want me, she's gonna break my heart i know it. Just like every other girl in history! Why do i bother! It's all so stupid!..."
"Well.....i'm just gonna write you a little reality cheque here...............theeere we go. And you get back to me when you've tightened that loose screw mkay?"