MagicDraw UML employs at least three types of model validation:

1. Preventative Validation: it is not possible to combine certain kinds of elements and relationships that do not agree with the UML specification, for example one can't make a Connection between InstanceSpecifications (one can however make a Link).
2. Active Validation: when constraints and validation rules from active validation suites are broken the tool flags errors in the  model "actively" with a coloured validation error box around a problematic element's symbol (the colour will depend on the severity level, a warning is yellow), and with a little red cross in the containment browser, and offers possible fixes.
3. Passive Validation: the rules are only applied if explicitly selected under menu Analyze>Validation.

Please visit also:

• The MagicDraw UML help pages in the tool, and search for 'validation'.
• The related (detailed) tutorial: 
  HOWTO use the active validation engine to apply fixes in the MD SysML Plugin