The proper way is to check it with a volt meter at the battery with the vehicle running. If you don't have one you can disconnect the negative battery terminal while the truck is running, if it dies you are not charging, if it stays running you are. Do not disconnect the positive, more potential for unwanted sparks.

1953 Chevrolet 3600
Fried Green T'mater (Vern)
In the Stovebolt Gallery
Caution: Political Statement
Man who runs in front of car gets tired, man who runs behind car gets exhausted.