You can tell by the way he looks at you when you are with him.
By the number of times he calls you just because.
How he makes you laugh when you talk together.
The way he noticeably lights up when he see you walk into the room.
When he remembers your birthday or a date that you mentioned before without you telling him.
By the way he holds you and kiss you whether alone or in public.
When he shares his credit status or his job salary with you.
If he does any and everything for you; you'll know.