I took mine to the local garden centre on a quiet day, onto the weighbridge and measured the weight whilst hitched(a) to the car, then took off the car and measured the entire weight(b). then (b)-(a)=towball weight.
Cracka
That is the best way to do it.
Remember, that tow ball weight is just 1 aspect and you need to make sure that you are aware of all your other weight limitations and what your trailer actually weighs - this can effect insurance and safety.
What Cracka describes above is IMHO a compulsary activity that we should all do.
ATM (Aggregate Trailer Mass) is what the trailer weighs fully loaded (including water and fuel) including the weight on the tow ball. To weigh this you need to unhitch the trailer from the car and make sure that the trailer and jockey wheel are on the weighbridge.
If you don't have brakes then your ATM is limited to 750kg.
The GTM (Gross Trailer Mass) is the weight transmitted to the ground by the axle(s). This is measured with the trailer attached to the vehicle (i.e tow bar is taking the weight of the drawbar), but ONLY the trailer wheels are on the weighbridge.
Your trailer will have a GTM limit based on the axle and suspension materials used.
Therefore as Cracker describes ATM - GTM = tow ball weight.
Tow ball weight should usually be around 10-15% of the ATM and also in accordance with the vehicle / towbar specifications.
Then you also need to worry about the GCM (Gross Combination Mass) which is specified by the vehicle manufacturer and specifes what the total weight limit of the vehicle + trailer is allowed to be.
Clear as mud !
Cheers
Camel Tow