If you already own the trailer here are some options:
Call an airstream/sob dealer and ask if they have a Trailer Tongue Weight Scale service. If you can't locate a dealer the scale is all over the net for sale.
Trailer Tongue Weight Scale: http://www.ebay.com/itm/Sherline-LM1...784#vi-content
Or just do some math, typically the tongue weight is 10% to 15% of the weight of the trailer.
A good weight distribution set will help.