How is known that Boudica existed?

"The Boudican revolt against the Roman Empire is referred to in four works from classical antiquity written by three Roman historians: the Agricola (c.98) and Annals (c.110s) by Tacitus;[2] a mention of the uprising by Suetonius in his Lives of the Caesars (121);[3] and the longest account, a detailed description of the revolt contained within Cassius Dio's history of the Empire (c.202 – c.235)."

https://en.wikipedia.org/wiki/Boudica