Prayer, of course. I always tell the Lord that "If this is not of you Lord, please shut the door; in such a spectacular fashion that even the atheists would know it!" I find its too easy to just amble along a path saying "well, this must be of God because things are going so swimmingly".
Nope, not always workin' like that.
Of equal importance is to have a good spiritual director, its too dangerous to rely solely on your own personal discernment. That's how Christian women end up with the legs in the air in abortion clinics. An outside pair of eyes, or two can help discern the situation without being clouded and biased by that romance induced vigor.
I have a couple of spiritual directors that I can call on when need be. I do have family members with strong spiritual gifts, but I've found sometiems they fall victim to their own biases towards me and what they want for me, rather than what God wants for me.
End of the day, I'm not really fussy about who I marry, as long as he's a good solid Catholic and hasn't done time for shooting previous girlfriends. People are too picky, have too high expectations. I don't even think "love" is required pre-marriage, sure, a certain level of tolerance and "like", but too many people think love is some butterflies in the intestine sensation where everything is just peachy. Love is self-sacrificial, it is about service and willing the good of the other above self. With that mindset, I think its such that love should increase AFTER the vows have been said.