Does anyone know what methods I could do to prove there is static electricity in a trampoline without having to buy anything?

Take two household object which recive different charges when charged (ooooo) and hold them both near the tramoline. One should attracted, the other repel. The repelling one will probably be more obvious. If this doesn't work, try charging a boolean from it, then see if it attractes your hair. (Come to think of it, see weather your hair is attracted by it!)