That's funny. Here, it's 87% relative humidity outside, and Shannon just bought us a dehumidier yesterday so we wouldn't get mould growing in our house. It's such a relief to have.

We also bought a cheap analogue humidistat ($7 at Crappy Tire) to monitor its effectiveness. You could try taking measurements of different rooms at different times to see how well yours is working. However, if you have forced-air heating, the best way is to buy one that gets permanently installed into the duct right beside the furnace.