Crystal Report Show hidden field if a detail record shows a number

Hi,

What would be the best way to show a suppressed textbox in a Crystal Report group header whenever any detailed record doesn't display a value in a particular field?

If none of the detail records shows a number in a field called mytestfield, the textbox in the group header should remain suppresed.

If, however, any or all the records in the detail section show a number in mytesxtfield, the textbox in the group header should show up so the user can see it.

Thanks.