It would be very difficult to determine the actual capacitance, as it exists between the outer surface of the topload and ground. However, it is most likely in the pF range. I would assume you want to know in order to be able to calculate the resonant frequency? You can measure that using a
Tesla Coil Tuner, which I think I posted in another thread recently. It uses a single 555 timer and some readily-available discrete components. It would definitely be a worthy build and will help you a lot in tuning your TC.
Regards,
Der Strom
P.S. Here's a site you can use to estimate the capacitance, as long as you have a standard topload shape (shpere or torus):