Well it’s between the priest in the confessional and the individual to determine who is in a state of mortal sin. We must remember that to commit a mortal sin, it must be a serious matter, that one consents to, in full knowledge that it is offensive to God and will to do so. There are millions of Catholics who have not been catechized appropriatly. I’ve met adult Catholics who have been taught by liberal priests and sisters that you don’t have an obligation to go to Mass on Sundays and Holy Days if you don’t feel the need. Some liberal priests have told people that Artificial Birth Control is a matter of their own concience and that thay don’t have to listen to the Papal decree, Humane Vitae. Having heard this from what were supposed to be competent authorities, they don’t follow the teachings of the Church on such important matters with no intent of offending God, so while they sommit a sin that objectivly is a mortal sin, it may be in some cases that their ignorance of the importance and the sinfullness, makes such offences venial.
That does not mean that we should not pray for them and encourage their education about what the Church really teaches. This and other Catholic web pages are one way of doing that.