Mix some coconut oil in with the powder. Place this Carribean bullet in at number 27 or something. When you smell a pina collada, change mags. Increase the powder load in a bullet, when it feels like you fired a C3 from your shoulder, change mags. The tracer one sounds simple enough, but I want more complication and less reliability. Maybe a coloured powder mix, every five rounds is a different colour. When you see lavender, change mags. All I remember from plastic mags was that they sucked, broke, stayed dirty, jammed, and sucked. Other than that, no problems with the old plastic mags. Maybe a little gauge that contains a 30th of its capacity with every shot from a rifle. After 30 rounds, your front sight pops a little pink nipple up. When you change mags, the pressure is released and the nipple drops.
Just wing it, when the time comes that you figure you should change mags, do it.