The system used for Dark Heresy and Rogue Trader is percentage based, so I believe Deathwatch will be the same.
Your chance of succeeding is stat influenced by (i) how many levels of skill you have and (ii) circumstances, e.g. Knowing something is Intelligence based so you would have a base chance equal to your intelligence plus, say, 20% for it being common knowledge, and plus a further 10% for having more than one level of the relevant Knowledge skill.
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