Option 1: When "All" is selected in a dimension filter

TOTAL( COUNTD( [Category] ) ) = SUM( { FIXED : COUNTD( [Category] ) } )
 

• The above calculation will return True when "All" is selected in the [Category] filter. ([Category] is a dimension specific to the Superstore Sample data source. )
• The TOTAL(COUNTD()) expression counts the number of values of [Category] in the view.
• The FIXED expression counts the total number of values of [Category] in the entire data set.
• When no dimension is included after FIXED then the expression will be evaluated over the whole data set.

Option 2: When "All" is selected in a parameter

1. In the data pane, right-click [Category] and select Create Parameter.
2. In the Create Parameter dialog box do the following and click OK.
   1. In the Name text box, type a name. In this example I will call the parameter "Category Parameter".
   2. Add "All" to the List of Values.
   3. Right-click [Category Parameter] in the data pane and select Show Parameter Control.
3. Create a calculated field with a name similar to "Category Parameter Filter" with a calculation similar to the following:

   [Category] = [Category Parameter]
   OR
   [Category Parameter] = "All"
   

4. Drag [Category Parameter Filter] to the Filters shelf. 
5. In the Filter dialog, check True and click OK.
6. Create a calculated field with a name like "All Selected (parameter)" with a calculation similar to the following:

   [Category Parameter] = "All"