“Savoir la vérité” is not a proper turn of phrase; using the verb connaître is the correct way: “connaître la vérité”.
In the case of abstract things such as 'to know the truth', both 'connaître/connaitre' or 'savoir' can be used colloquially -> see link here: connaitre-vs-savoir-to-know-something-to-know-how-to-do?
I hope this is helpful.
Bonne journée !
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