catch someone red-handed
US /kætʃ ˈsʌm.wʌn ˌredˈhæn.dɪd/
UK /kætʃ ˈsʌm.wʌn ˌredˈhæn.dɪd/

1.
to discover someone in the act of doing something wrong or illegal
:
•
The police caught the thief red-handed as he was trying to break into the car.
•
My mom caught me red-handed eating cookies before dinner.