Positive? or Negative charge? How to tell on a Van de Graaff generator?

[Mike Johnson]

My son and I built a small Van de Graaff generator & we've been playing around with it. We get sparks up to two inches. We've done the fluorescent light thing, and built an ion motor whirligig, and all the usual stuff.

The science fair is coming up, and he wants to test whether the charge building on the top sphere is negative. I know VDGs can be built either way, and we can use the triboelectric series to predict whether the top sphere has a positive or negative charge based on how we built it. But what's an easy way to see which is the case? All the stuff we've done (the fluorescent light, the ion motor, etc) work just as well hooked up to either end of the generator.

Any ideas?