A figure of speech as a replacement of 'evidence' or 'proof'

dead to rights

[UK bang to rights]

in the act of doing something wrong or illegal

Cambridge

: "Dead to rights" is seen from the 19th century on, and is used mostly, I believe, as a synonym for red-handed or in the act, or at least with positive evidence of guilt, as in "I caught him dead to rights," According to the OED it can also be used to mean "completely, certainly." [emphasis mine]

Phrase Finder

"We have her dead to rights, it's all we need to prove she did it.

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something which definitely shows that something else is true or correct : definite proof

The photograph is proof positive that the accident happened the way he described.

[Merriam-Webster]

Consider In flagrante

Also in flagrante delicto¹

(Latin: "in blazing offence") or sometimes simply in flagrante (Latin: "in blazing") is a legal term used to indicate that a criminal has been caught in the act of committing an offence (compare corpus delicti). The colloquial "caught red-handed" and "caught rapid" are English equivalents.

*We have her in flagrante, it’s all we need to prove she did it.

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¹Wikipedia