If you can buy a noseweight gauge from a caravan accessory shop, so much the better. If not you will need something like a set of bathroom scales and a short length of wood. The length of the piece of wood depends on your tow ball height, basically you need to weight the hitch at the same height as it will be, when hooked up to your car. So, you will have your tow hitch resting on a piece of wood, resting on your bathroom scales.
The process of actually measuring the nose weight is simple too. Once you have loaded the van make sure the handbrake is on (or chock the road wheels) and wind up the rear steadies, then wind down the jockey wheel (lifting the front of the van). Wind up the front steadies, so that the caravan is resting on it's axle and jockey wheel. Place your bathroom scales and block of wood/noseweight gauge under the tow hitch, then slowly wind up the jockey wheel so that the front of the caravan lowers itself onto the scales.
It may take a little to a fro'ing with the front steadies before you can get all the weight on the hitch, down onto the gauge, but at the end of it all the jockey wheel should be slightly clear of the ground, with the scales taking all the weight from the hitch. Take a look at what it say on the scales and that's your reading.
Remember, if you are not happy with the weight, lower the corner steadies if you need to get back into the caravan.