$test = "[[*url]]"; if ((isset($test)) and ($test != '')) { echo "sometext"; }
[[*MyTemplateVariable:notempty=`This TV has content`]]
http://rtfm.modx.com/display/revolution20/Input+and+Output+FiltersLink is broken, use this instead https://rtfm.modx.com/revolution/2.x/making-sites-with-modx/customizing-content/input-and-output-filters-%28output-modifiers%29