The way it works is you plug it in, and while it's charging the red LED will flash on for a moment about once every 15-20 seconds. Not much longer than the blink of an eye. When it's done charging, it no longer flashes on.
What's really wrong with this is that in order to tell if it's charging or not, you can't just glance at it and see, you have to watch it. And even if you feel like you waited enough for the flashing light, maybe you blinked and missed it, so you have to wait longer.
A better UI would be to simply show the light while it's charging. Of course that has the downside that maybe if the light doesn't show it means it's not plugged in, or the switch is off.
Even better is it could flash more rapidly while it's charging and then stay on steady when it's finished. That way you can easily tell. Or two colours, one for charging and one for charged.
The way it is now stinks.