Take look at this example:
https://github.com/m5stack/M5StickC/tree/master/examples/Advanced/AXP192/PowerWake

function 'M5.Axp.GetVapsData()' can be used for what you need. When value is above 4.5V it's charging.
function M5.Axp.GetVbatData() will show you battery level.

@drwino To conserve battery power you can try disable backlight on lcd with :
M5.Lcd.setBrightness(0);

update: I just tested this:
M5.begin(false);
backlight almost disabled and nothing on screen.
so better is just set backlight to zero.