All red is empty.
Take the insert out of the shell, and push against the silver blade spring on the feed spool side.
That pushes against a feeler rod, which (through a rack and pinion mechanism) rotates the indicator dial.
Later magazines have a round 'column' with rolling top that pushes against the spool, instead of the blade spring.
If nothing happens when you push, i.e. the dial doesn't rotate, the thing has become dislodged.
It is one of those things that is not difficult to fix, but you must know how, and it is difficult to explain just that.
The top, where the retaining arms/bar (depending on age of the back) are, must come off, and then you can reposition the thingies. Putting it back together again is a bit fiddly (the rod that is connected to the key and locks the insert in the shell must be repositioned), but doable.
The indicator, by the way, is not a precise one.
It goes from red to white/silver as the feed roll diameter reduces. But apart from telling that there is something pushing against the feeler behind the spring, which is getting thinner, it provides no more detailed info.
So something that you could live without, i.e not something that you necessarily need to get fixed in a hurry (if at all).