Once a document is opened, there are several ways to validate a document in CSS HTML Validator:

Press F6, or

Select Validate->Validate Current Document from the main menu, or

Bring up the editor window's or Results Window's context menu and choose Validate, or

Press the Validate Document toolbar button on the Main Toolbar or Editor Toolbar, or

Press Alt+Up or Alt+Down if the document hasn't already been validated, or

Click on a panel in the Results Window that states that clicking on the panel will validate the document.

After validating, the validation results will appear in the Results Window.

See also the Validating Local Documents and Validating URLs topics.

In the standard and higher editions, there are also validation modes you can choose from.