Yes, that is a good line indicating his doubt - but also there is the line just before that where he reveals that he did not 'sense Luke'. Sidious had to wonder why that might be. Either he thought Vader was wrong (and thus he would doubt Vader) or he thought Vader was right - which could still lead to his doubting Vader because he would figure that Vader was sensing him due to some connection to the force that was not entirely at one with the dark side. That is, it was distinct with all of the dark side sensing that Sidious was using. Although, Sidious could have merely concluded that their father-son connection was the reason, having nothing to do with light/dark side at all. But I do think that Sidious was smart enough to realize that Vader had problems from the start right through the end in his ability to totally get rid of the Jedi inside. You have to imagine that if it really did exist - even a spark - there would be moments when Sidious would suspect it. You know in the movie, one would surmise that Vader found out about his son from Sidious. But in the novels, Vader already knows about Luke. The latter makes more sense because of course Vader would want to know who the force sensitive kid was who had taken out the death star. But I don't know if that is Lucas intended when it came to the movie because he has Vader say "but that is impossible!"