How do you know if you really love someone romantically not just as a friend?

He or she likes you as a friend. If it was romantic, you would know. If you have romantic feelings for the person, the relationship must be painful in some ways. The best thing to do would be to get the issue out in the open, either by talking about it or just asking them out on a date. At least then you have some indication as to the direction things may take.