Here's a flash app that you can test your hearing , how high it goes at higher frequencies. Best to start at the top and then notice when you can start to hear something. For me, it's 18 000 Hz which is pretty good considering how bloody old I'm getting. How about you?
This all came from a shopkeeper in england who used high frequency sound to repel teens . Fascinating and clever idea.
Oh by the way, you need a decent set of speakers or headphones to do this, otherwise you can blame your crappy speakers :-)