Look on the door shuts/under bonnet for the ID plate, this will give the gvw (gross vehicle weight) which is the heaviest the vehicle can weigh including everything (you as well!)
Put all your gear into the van, take it to a local public weighbridge and have it weighed with you sitting in it, plus a passenger if you regularly have one. They will give you a certificate showing what the weight was. The difference between this weight and the gvw is what you can still put in the van. A litre of water weighs 1kg so if you have 500kg still available you can fit in a 450lt tank (allowing for the weight of the tank and fittings)
This is so you are still legal
However it's never a good idea to work right up to the limit - shortens suspension life amongst other things.
Forgot to mention: fill the fuel tank too before going to have it weighed
