Using tool validation to create parameter based on fields of another parameters input?

I'm assuming I'm missing something and this is a pretty easy thing to do but I'm stumped...

I'm working on a script that currently takes and input excel file and uses a "hard coded" field name that I know is in the table. I would like to make the tool a little more user friendly in that I would like to have a drop-down of field values from within the table, letting the user select the appropriate field from the table. This would ensure that the field being used is indeed in the table.

So... If I have an input parameter for a file in which the user selects an excel workbook, how would I create another input parameter that would then be a drop-down menu of field headers from the selected excel workbook. Is there some kind of validation code for this?