S T R U C T O R I Z E R - User Guide
Settings > Analyse structogram?

In menu "Diagram" or by pressing F3 you may disable or enable the Analyser feature, which performs a "live" static analysis of your diagram with respect to syntactical, structural or certain semantic issues:

Diagram menu item Analyse

In the Analyser Preferences you opt in or out specific rules of the analysis. See Analyser Preferences for a list and a short explanation of the rules.