By looking at the indicator on the left hand side ('inside' the key that is used to unlock the insert).
It is linked to a feeler, sensing the diameter of the film roll, and shows white if there is film inside, gradually changing to red when the roll's diameter decreases as film is transferred to the take up spool, letting you know how much film is left on the roll.
Mind you, it is rather course mechanism, and the indication is not very precise. The frame counter on the other side is an exact indicator of how much film is left.
If the frame counter is beyond 12, the indicator on the left showing red, its still best to give the wind crank on the magazine a number of turns, to make sure that if a finished roll is in the magazine, the trailer is wound around the roll before you open the thing.
You could use the wind crank on the magazine. Give it a turn and feel whether it is pulling film through or not. But that requires you to be able to tell by judging the amount of drag/force needed.
And you'll get spacing problems on the rest of the film, if there is one inside.
You do not have to open the insert to see whether there is film in the thing directly.
Opening the insert will expose lots of film, and reset the frame counter and spacing 'mechanism'. So if you do that, notice there's film inside close the thing again, you will have wasted a few shots, and will have spacing problems for the rest of the roll.
A better way is to take the slide out, in dim light, and see if there is film in the film gate. That way, you will only waste one frame.
However, and concluding: film is cheap. If you lost track, just use the wind crank to fully wind any film that might be inside the magazine to the take up spool, remove it (or find that there's nothing inside), and put in fresh film.