Eliminating double scheduling

Mark,
Long time, no talk to!
I do not know what happened... the post saved before I finished...

Need this functionality on the Travel Form:
On the save - check the following:
Employee ID (on form) - listed on Travel Table
Do the Start Date & End Dates (on form) - correspond to dates on the table

If these are "true", the record cannot be saved with a message like this "The associate is already scheduled on a project in this time frame"
If "false" the record can be saved.

I am just not sure where to start on this...

Thanks for your help!