With this standard format, a zero will appear as as 0:

#,##0;-#,##0

However, you can set zero to appear as "na" using the following
format:

#,##0;-#,##0;"na"

My question is whether it is possible to test whether the cell is
showing a 0 as "na", even though the underlying value is still a 0.
For example, I cannot use IF(A10="na",... because the cell A10
actually still contains a 0; it just looks like "na" because of the
formatting.