Put your cursor in the field, in Browse mode, and choose Insert menu, then From Index. That will show you a list of unique values in that field, from all records in the table.
In you case, since nothing except "yes" or "no" makes any sense logically, you could just do a Find for "yes", and a 2nd Request for "no", and Omit. That would be all records which have neither yes or no.
THANKS!!! Insert >>from index... is exactly what I was looking for! It shows you all of the different values in the field.