So if they don't stop sinning, ipso factso, they're not elect? I would guess it's more a matter of degree.
Those who claim to have recieved the new birth but produce little or no fruit are certainly suspect. If the New birth is not life changing, it is reasonable to suspect it has not taken place. Offcourse only God knows the heart.
Is there some general guideline on this? Or is it so long as they are contrite; IOW is everyone who is regrets their sin, elect?
Only those who repent and place they Faith/Trust in Christ's finished work on the cross as their only salvation.
Is everyone who thinks they're elect, elect, or could they be wrong?
Yes, we are told to exanmine ourselves to see that we are in the Faith. If the fruits of a new life are not present or we are found to be trying to score points with God for salvation, then most likely the new birth has not taken place.
Fox guarding the hen house?
Could you be wrong in your examination on whether you "pass" and are really already saved?
I'm being a bit facetious, but when looked at properly, as I think you are doing, in practice, personal experience of double predestination is a bit tenuous.