[T2] Alternator charging?

[Richard Jack]

How can I test if my alternator is charging?  

1974 bus 1800cc. All stock.  

I see the regulator/relay?  Mounted on the firewall, where the alternator wires are plugged in.  
Brown, red, blue, and green wires.  

Could I test it there with a volt meter?  Which wires do I test?

Thanks
Richard
Lubbock, Texas.