My twin girls just turned one. One nurses mainly when she's tired and the other anytime I offer. How do I tell if they nurse for comfort or nutrition?

Looks like you know. It sounds from your description that the one who nurses when she's tired is nursing mostly for comfort while the other needs the nutrition. I don't know other ways to tell for sure. But i see nothing wrong with nursing for comfort; to me this is just as valid a reason for nursing as to supply nutrition.
Recommended age. for weaning is as close to a year as possible. Wean to an open-mouth cup to promote lip closure for speech. Limit milk & milk products to 16-21 oz/day. Brush their teeth after feeding or nursing before they go to sleep. Their first visit to a pediatric dentist is due now. Once they are 15 mos. developmentally, they will protest longer & louder than if you wean now, but the choice is yours.