poświadczyć in inglese:

I can vouch for his reliability.

Sorry, I can't vouch for Anna because I've never met her.

The fingerprints on the knife attest to her guilt.

His success attest his diligence.

This lady witnessed an accident at three o'clock this afternoon.

He witnessed the accident on his way to school.

He witnessed the accident.

She witnessed him being killed.

Strange isn't it. By the schedule around now we should have all witnessed an UFO, and united in researching. "From 'witnessed' it's never gunna happen."

Lucy witnessed a murder in the main square of Florence.

The day before yesterday he witnessed a weird incident in the wilderness.

The festival witnessed an unusually large attendance.

The two races witnessed numerous revolutions during their time on the planet.

I asked him about what he witnessed, but he wasn't very forthcoming.

She witnessed the truth of the statement.

The letter has been authenticated by handwriting experts.

Works believed to be by Banksy failed to sell at auction after the artist refused to authenticate them.

authenticate (in general) prove something is true or genuine

Said it was 18th-century Britannia silver, so I thought you might be able to authenticate it.