The only foolproof way of knowing is to go by the GVW - They may have changed the terminology but it stood for Gross Vehicle Weight. This is the absolute maximum the vehicle can weigh on the road. Also sometimes called Kerb Weight.
It will be stamped on a plate somewhere, on a door pillar, under the bonnet etc.
You need to put everything you carry in or on the van, fill the fuel tank and take it to the nearest public weighbridge. For a small fee they will weigh it for you. Make sure you are in the van, and a passenger if you intend to carry one. Leave your water tank empty.
When the van is weighed you will get a certificate from the weighbridge operator. Keep this to show any official who may ask about your vehicle weight.
The difference between the weighbridge figure and your GVW is the amount of water you can carry.
So:
(Say) Vehicle GVW is shown as 2,500 kg
Van/full fuel tank/ladders/pump/Di/reel(s)/hoses/poles/driver/passenger etc etc weighs 1,800 kg
This gives 700kg spare capacity so you could carry 400lt water with a safety margin of 300kg. You could carry 700lt but it would shorten the life of your suspension!