There are two ways.. First the easy way
http://www.rvsafety.org
Just visit the link on the left side of the page for weighing your rig, they come to you, set it up and provide a bunch of info CAT scales do not, Price is roughly the same
Method 2
There are two (well 4) kinds of truck scales.... We are not so much interested in segmented (Weighs each axle like the CAT scales do) or not (Wehighs the whole rig) as we are Bridge or Flat,, Many of the CAT scales are bridge scales and those just won't do.. You need a flat scale
With a flat scale if it's segmented you drive on normally, centered, and get weighed, , Then you drive so only one side of your rig is on the scale, the other on the ground.. This gives you the weights for those corners,, Subtraction gives you the other corner.
If it is a single (non-segmented) scale.. Drive so one axle is on, then the other, then one wheel, (Front) and then the rear (Same side) and do the math to get the other side once again.. You end up paying for four readings that way and might well find the link above less expensive.
To find flat scales check junk yards,, Farmer's elevators and other places which might be listed under SCALES in that book your fingers do the walking through.. Hint; The pages are yellow.