Type:Boolean #x formatter

From semantic-mediawiki.org
Help:Datatype "Boolean"Type:Boolean #x formatter

A special formatting symbol for Booleans is the lower-case letter x. When given as a text for displaying results, SMW will use an X that resembles a cross for true.

{{#ask:
 [[Category:Project]]
 |?successful#x
}}

This would show a cross for all cases where the property is set to true, and nothing if it is false or unset.