I would need it to just return a value of 1 if it were touching something, or zero if were not. I would also need it be small and thin, with a short read range. What would you recommend?
I am unaware of anything that would meet your specified needs. There are passive RFID sensors that can detect temperature, pressure and moisture levels, but I don't know of any sensor that could detect whether or not something was touching it.
RFMicron makes a chip that can detect changes in an antenna's impedance. This could be used to detect if things touch the antenna, because items with a high enough dielectric constant would change the impedance. But if something with a low dielectric constant (dry cardboard, for instance) were to touch the antenna, this could not be detected.
—Mark Roberti, Founder and Editor, RFID Journal