It is objectively sinful to receive Communion without believing that one is really, truly, and substantially receiving Christ. However, those who are innocently ignorant of this fact will not be held accountable for it. Those who are culpably ignorant, on the other hand, are sinning in such a way that they are liable to judgment. They are committing either a venial or a mortal sin depending on the level of culpability for their disbelief.
It is uncharitable to assume that people are sinning. Paul tells us to reckon that others are better than ourselves (Phil 2:3). A good way to bring the subject up would be to talk with people about how much Christ’s Presence in the Eucharist means to you and what a profound and wonderful teaching of the faith it is.