check the backs of the buttons and see if there's a chanel stamp or mark on them. not foolproof, as i've had authentic chanels with buttons that weren't stamped on the backs, but if it's there, chances are it's authentic.
the lack of fiber/size/season tag is usual. ever since chanel started putting those honking big fiber/etc tags below the labels, people cut them off. they're itchy and bothersome. i think i've only had a few that still had them on. i believe that started either in the 80s or later 70s, and onward.
i've never seen a denim chanel before, but then denim was awfully big in the 80s and it would work well for spring/summer collections.