I think that Expedito Bipes's answer is spot on, but as is usual with Latin, there is more than one way to say it.
Sententia veritatis is another way to say it. It perhaps carries a slightly different connotation, leaning a little closer to "opinion of the truth", and maybe not as close to "understanding of the truth" as cognitio veritatis.
I'd also like to note that verum, literally, "a true thing", can also be used to mean "truth". So cognitio veri and sententia veri are also alternatives.
Another note worth mentioning is an idiom peculiar to Latin (as opposed to English) in that an abstract noun of a verb can often be replaced by a participle. So verum cognitum or verum sensum can be understood to mean the truth as conceived or the truth as learnt. If you go this route, you'll need to rely on context to tell whether verum cognitum means "conception of the truth" or "a truth conceived".