Locating focal point on offset dish?

I'm a Physics teacher and am in possession of two old oval DirecTV dishes (24x18 and 19.5x18 in).  Neither has the receiver attached, so I don't quite know where the focal points are.

Is there a convenient way of determining the focal point?  I'm not looking to receive satellite signals, instead, I'm looking to do some sound experiments.

Any help would be appreciated.  (feel free to email me)

Thanks.