How can I tell a string aggregation is ambiguous

I have a measure and it is aggregated from a measure in lower intersection (string measure A at dept_str_plcl intx is aggregated to string measure B at dept_plcl intx), String measure A has a default_agg as ambig_pop. Some string measure B is ambiguous ("?"). I need to write a if statement to evalue the measure B. How can I do that? Thanks