How Do I Set Form Field =Required property ONLY if another field is filled?

I have a form where I would like to set the field required property ONLY if another field is filled (specifically if someone enters data listing a serial # for a one of the units, all pass fail data for that unit becomes required) There are multiple units that can have data on a form, but not all WILL have data based on test set up, so I cannot make all fields required.

Thanks!