I'd like to ask a related question:
If no design or load ratings for the existing structure are known, how will (or should) the rigger approach the job? How does a rigger evaluate whether or not the structure is adequate for the load to be suspended? At what point does he call in a structural engineer before completing the rigging?
Brian, The simple is answer is without a structural assessment you don't know.
The design load of the trusses or beams may already be exceeded. (everyone keeps adding equipment, AC and other heavy things)
It is possible an assessment has already been done (As far as hanging equipment) during the building planing stage of construction.
If it is an old school, that information may not be available.
A lot of things get "bootlegged" in without any problems, However I would recommend NEVER doing this or be part of doing this.
I really don't even know how many spaces we have worked in that someone opened up 2 or 3 spaces to create 1 large space.
They determined the "Partition" walls were "Non Bearing" and removed them only to find out that they removed "Engineered Sheer" Walls.
This is how rectangles become parallelograms
Almost always more expensive to fix than to have done it correctly in the first place. (by a factor of 2X or 3X more).
Just my thoughts.
Regards, John
To answer the OP question, I am guessing your goal is not to just hang a piece of truss.
What is the load that will be hung from the truss, Lights, Wires, Speakers???
The problem as I see it is not having someone (Structural Engineer) with the Global Picture putting all the pieces together.