Attach a couple of pen tips to the holes at the end of the low frequency tuning fork.  Start the fork vibrating and draw across the top of the transparency.  Practice will be required to get a good trace.  The trace should show that the two tines of the fork are always in opposition when vibrating.  For instance they are always moving towards or away from each other.

You can also observe tuning fork motion by holding a vibrating fork in front of a television screen or computer monitor.  The television or monitor will act like a slow cycling strobe light and the forks will look like they are bending.