A label in the coach stating it is solar ready. The label will generally be in the area where the wires for the solar controller are located. You can also check the electrical diagrams for your coach to see if the wiring was installed.
What year coach? I see nothing in any FW 35SD specs stating it is solar ready or prewired for solar. I believe, and I could be wrong, the "solar ready" feature did not start until the Magnitude/Omni change over.