You will never truly be empty as baby can get a few drops, but when they say finished, it generally means when the flow really slows and they are not getting much other than a few drops, baby will usually say when this is by coming off the breast. You usually offer the other breast to see if they are still hungry. If they take it then that's fine if not they should be satisfied from one breast.
You will however get to know your own baby. Mine was fussy at the boob for the first few months and would bob on and off a few times every few mins, so I would keep him on the same boob until he made it more obvious he was done on that side (refused to relatch) . Then swap him to the other side. Turns out he was a bit lazy and liked the initial few mins of the feed as this was the sweeter and faster flowing milk!