you can do the non smart way test it like I did(not recommended but I didnt really care) start car, pull the positive cable and see if the car stays on.
On new cars doing this will turn off the car as there are failsafes, but not in the e46.
take it to an alternator shop/ independent
"This vagabond life can I transcend? To give up sleeping beneath the stars. Life on life’s terms does not pretend.
This journey fades as I near the end. The long sleep under Venus and Mars.."