Adrien sat there at his desk in his bedroom writing out his checklist. He was listing all of Ladybug's physical, personality, and character traits that he could think of, to see if he could determine who she was under the mask. And as he went over the list, one name popped up and wouldn't go away.
Marinette Dupain-Cheng.
Everything matched up perfectly. LB was Marinette, no doubt about it.
The question was, what to do about it. He could ask Ladybug, but she'd deny it. Besides, where's the fun in that? So, in true kitty-cat fashion, he devised a sneaky ambush to out her.
A few days later, they meet to go on a patrol together. Chat gives her his famous cheeky grin and says, "I figured out who you are LB, under the mask. I figured it out."
"Oh yeah?" she responds. "And just who do you think I am?"
"Let's just say that I have your back in battle and you have mine in school."
He watches the wheels turning in her head until the light dawns.
"A-Adrien?"
"Gotcha!"
An angry Ladybig advances towards him, and he starts to back up.
"God Dammit, we aren't supposed to know each other's secret identities! You know that!"
Chat looks at his wrist as though there was a watch on it and says, "Oh dear, look at the time! Gotta run and do the patrol!" And he off he goes, laughing his head off with a pissed-off Ladybug in hot pursuit.
She caught him in about 15 seconds. It's hard to go fast when you're laughing like hell. And even harder when you're hanging upside down from the Eiffel Tower, tied up in a yo-yo string.
Being in love with a superhero does have it's downside sometimes.