For example, he may refuse to divulge his last name and for many people this is often a dead giveaway that the guy is married.
While it is normal for people to hold back initially, if the guy has something to hide, he will usually not reveal details about where he lives, what he does for a living, his family, his upbringing etc.
She may perceive it as his commitment, an investment in their relationship.
It may feel precious—something to be guarded and protected, nurtured. The friends she's told will remind her, “He’s married.” Worried family members will ask, “Is he still with his wife?
I’ve met many charming men who have been very nice to me.