if you try to figure out how you might be deluded, you are more likely to end, or at least decrease, the delusion than if you do not.

Yes, I agree.

How, from the inside, could you tell yourself apart from them?

By carefully attending to things related to whatever it is that is like Christianity to me in this example, and carefully deriving my beliefs from those observations.

If I saw a Christian who was doing that, I would not encourage them to worry about being deluded; I would encourage them to keep doing what they're doing. And if I saw a Christian who was worried about being deluded but not attending to their environment, I would encourage them to worry less and pay more attention.

And the same goes for a non-Christian.