Sorry, but there's no such thing as "dead ends." This is just a big false saying-the hair itself has no life, because if it did, it would hurt to cut it. The only living part is the hair follicle, found in the scalp. The hair shaft itself is dead. Your roomate probably just said that because your ends were frizzy. T
There is such a thing as dry ends and split ends. I don't know how you can tell if you need to trim, because everyone's hair is different. For me, when I used to hot curl, I knew i needed a trim if my curls fell out shortly after curling my hair. Now, I just look at the ends. If I find more individual split ends than I can cut individually (if there are way too many) then I just get a trim straight across.
Don't listen solely to people telling you that you need a trim. I had a woman at work tell me that, and I had just had my ends trimmed a month before-I haven't used heat in well over a year, so there was no reason for my ends to be split. My ends don't dry super silky because I air-dry...I guess they looked bad to her, but in all actuality, I didn't need a trim. If the ends are broken off, it probably is best to get a trim. Try to moisturize first, and pay close attention to your hair. If the ends are excessively dry or split, get a light trim to start fresh. Do it if you think so...not your roomate.