When he seems to be overjoyed when he meets her and when he spends time with you.
When he gradually welcomes her feminine stuff in his house; her special soap in the bathroom, her earrings on the table and her tampons in the bathroom.
He makes long term plan about sharing his activities with her. He will make sure that his girlfriend can participate in his social activities.
When he never really thought about getting married or having children and he wants those things with her
He will take care of himself financially, physically and spiritually to make her feel secure and happy.
He will wake up before her and makes sure that she is okay, wonders if she is hungry, checks to see if she is warm enough or if she would need anything else.
He constantly tries to impress her; he will pick her up and brings her home, brings her food, offers to pay for some activities, gives her surprises?
Other temptations are not a threat. He is just not interested in any of those distractions
She will have no doubt in her mind. She won't need to ask. She will feel it in her guts. If she doesn't, and still remains with him, she is settling for less
