If it's a commercial vehicle there will be a plate somewhere with the GVW (gross vehicle weight) and GTW (gross train weight - total weight of vehicle and trailer)
The GVW is the maximum the vehicle can weigh, including everything (payload/driver & passenger/fuel/tools/spare wheel etc etc)
Put everything you would normally carry in the van, take it to the nearest public weighbridge (look in Yellow Pages) and have it weighed with you, and a passenger if you normally have one, sitting in it. Take note of how much fuel there is in it and allow accordingly.
The difference between the measured weight and the GVW is how much more you can put in it.
This is the only sure way to know if you are overloaded or not. I keep the weighbridge certificate in the van to prove I haven't overloaded.
Cheers,
Ian