Best way is to get your total arrow weight is to weigh them, for sure. But, if you don't have a good grain scale handy and the pro shop is a pain to get to, there's another option. Go over to
www.bowjackson.com and use the arrow weight calculator he's got. It won't be exact, but it'll get you in the ballpark.
Jackson'sgot a lot of other nifty calculators and stuff over there. His site should be in every bowhunter's list of favorites.