The answers here are so funny :-) and yet serious :-)
I think the key word in the question is the word "tell".
There is no one formula. Only the woman can TELL if her man is into her IF she takes the time to observe him, get to know him better, find out how he behaves when he is so into something like his passions (like how his eyes light up when he sees a new model of the car or gadget he loves -- does his eyes light up the same way or more when he looks at his woman), his causes and his dreams.
People love in different ways and each man may show their love in ways that are different from another. But they do not love any less. Just differently. But if you are the woman and he is the man you love then your heart will TELL you and you never need to ask this question.
Constantly comparing him with another man will only make the woman's life (and the man's) more miserable...after she has already chosen him amongst others.
This question then is most useful and will be most beneficial when the woman is yet choosing the man for her. It will take time and effort to get to know him better so she can TELL for herself if he is really the guy for her. And it will be different for every woman. So no cheating and copying from each other, gals :-)