In the Stack List, scroll down until you see a unit that looks like a horse pulling a cart and double click it.

Hopefully, you see something like this. I call this Army Information Window. From this window, you can get all the information on this particular caravan. Right now, concentrate on the “Load 50/50” in the upper right hand corner, and the Load Column in the Commodity Chart.  These tell you that the unit can carry 50 commodity units and it ain’t carrying anything right now. Click on the Load Button.