Are you referring to the asterisk that is shown when using Data Blending (multiple data sources in a single visualization)? The asterisk is Tableau's way of informing you that there are too many values to bring in from the secondary data source at the current level of detail, and Tableau must treat the result as a NULL. The asterisk is really just a visual representation of this special form of NULL, so you can write a calculated field to detect NULL and report a string message. For example:
IFNULL(ATTR([secondaryDS].[secondaryField]), 'Secondary field has too many values')
Does this help?