Hi Mirco,
thank you for your very good question, and apologies for the late response (I was on holidays).
In fuzzylite 5, the only way seems to be computing the activation degree again (which is nonsense, and I will fix this). Please, check the method Rule::activationDegree(const TNorm* conjunction, const SNorm* disjunction). The conjunction and disjunction operators you get them from the RuleBlock in which the rule is registered.
Cheers