All they have to do to prove it is disclose the financial records for the conference showing that no taxpayer money was used and that charges for the rental of facilities were not reduced -- contracts and agreements signed, invoices paid, and other financial records would be acceptable documentation. Those were public facilities that GLSEN held their conference at.
GLSEN was a paid consultant to the state DOE. GLSEN was paid with taxpayer money to develop programs for the public schools, as documented here and here. Since they were paid by the state with taxpayer money, GLSEN's financial records for the period in question should be made available for public inspection.
One of the most disgusting aspects of taxes is that they are used to fund things that an overwhelming majority of taxpayers would oppose.
Shalom.