There are a lot of unknowns in your post. Can you upload a sample workbook that shows what you have?
You don't need to upload your entire workbook. Pare it down to give enough data to show how you use [A]. Maybe a few other dimensions, and a bogus measure or two.
Follow the steps demonstrated in the video linked here:
Make sure the data has some of the multiple-value examples for [A].
I was able to find the answer myself through What ATTR() Does and Testing Whether ATTR() Returns * (Multiple Values)
Basically, I created a calculation called "Has Multiple A Values" like this:
THEN Null //if each value is null, then null
ELSEIF MIN([A]) != MAX([A])
THEN 1 //multiple values found (ie: 2, 4, 5)
ELSEIF COUNT([A]) != SUM([Number of Records])
THEN 1 //multiple values, including null found (ie: 2, null, null)
ELSE 0 //single value found
And then I use that calculation in my table calculation
IF [Has Multiple A Values]