He will tell you that he’s recognizing you as on (arm extended down the line) or off (arm extended down the sideline). If you’re close they’ll usually even say “you’re on” or “you’re off.”
Sometimes if you’re in the between zone they’ll almost ask you your intention. You signal that you’re trying to be on (arm extended toward them) and they’ll nod and put their arm out toward you. Or you signal that you’re trying to be off and they’ll confirm or shift you back.
So looking at a pic and seeing if they’re a yard off or a hard and half doesn’t really say much. The ref will hold you accountable to what he signals to you.