on newer transits (post 2001) the payload is easily identified by the number on the side, mine is a t280, 2.8 tonne, the van weighs around 1.8 tonne so can carry 1 tonne
Most transits weigh between 1.5 and 2 tonne, here is a brief estimate of the numbers and weights to the best of my knowledge, I may be wrong though
old style new style
80 260 - 2.6 tonne, can carry 800 kg
100 280 - 2.8 tonne, can carry 1 tonne
120 300 - 3 tonne, can carry 1.2 tonne
150 330 - 3.3 tonne, can carry 1.5 tonne
190 350 - 3.5 tonne, can carry 1.9 tonne
As I was writing this I did a quick google search and noticed that there is a transit T430 available that will carry 2.3 tonne, it weighs 4.3 tonnes fully loaded so you will need grandfather rights to drive it (have passed your test before 1996 or 1997, not sure which date.