There’s no function to search a parent form for any values contained in a related form.
However, you can workaround this by using either a Script field or a Calculation field.
On your form B, add a Calculation field. Edit the formula and choose a field from the list of fields that show up that are contained within the parent form A. Any field will do. Then you can add a Saved Search that checks to see if the value of the calculation field is empty or not.