The last two things you mention are the most important and helpful: spiritual direction and prayer. But, as you realize they are not enough. Your friends reasoning is good: there is nothing like a live-in experience in a seminary or convent to really test your vocation, especially if you make it long enough for the initial novelty to wear off and give yourself the chance to experience the reality of the life you would be committing yourself to.

One of your fears seems to be that maybe your attraction to the contemplative life is purely emotional, you feel great when you are there to chant Office, but at the same time you realize that there is much more to convent life than that.  And since you can fantasize just as easily about married life, you know that being able to imagine yourself there with the nuns is not enough to know you have a vocation. All of this tells me you are of pretty sound judgment.

Your spiritual director and the convent vocation director will be able to tell you if they think it would be prudent for you to enter and test your vocation as a postulant. If they do, then the only really sure step you can take towards knowing if it is your calling is to try the postulancy.  It may not be necessary to sell everything in order to do so; perhaps you could leave your things in the care of someone you trust with indications of what to do if you are accepted into Novitiate or when you make your profession.

For now, while you look at this more closely in prayer, you could perhaps make some adjustments to your present life-style that will help you test yourself and also prepare you.  If you dont already have a fairly fixed daily routine, start now. Give set time to prayer, include Mass every day, Rosary, spiritual reading, time for personal prayer before the Eucharist, go to the local convent regularly for Office. If you are used to working with background music give it up for a while in order to experience silence (see how you work that one out with your roommate).  This may mean cutting back on other things you like to do, but I think it will help you to live part of the reality of religious life before taking the step: the voluntary giving up of many normal and good things because God is asking something else of you.  This way you can test your resolve and your perseverance, it will help you grow in intimacy and love with Christ, and you will be better prepared and know yourself better when the time comes to take a step. 

I hope this helps, and I'm keeping you in my prayers. 

God bless,