Not charging with headlights on

I've been reading this whole thread and wanna thank all parties involved for great information. I'm new to cars and am having a charging issue with my 73 Dart with a 440. Looking forward to using the above info for troubleshooting my own charging system.

How would you test if the alternator is good, but too small, considering I was told my electric fan pulls 30-40 amps on its own? My alternator is a 60amp

Thank you