After turning on the Error List, the MS Visual Web Developer 2010 can report warning/error in the panel automatically.
However, I would like to know what I can do to trigger this validation manually rather than wait for a unpredictable period until the application starts to check.
If you go to Tools > Settings on the menu and tick Expert Settings, the menus will now have a lot more options, one of which can be found in Edit > Advanced, Validate Document which should provide what you want.
If you want to assign this a keyboard shortcut, go to Tools > Options on the menu, tick Show all Settings in the bottom left of the Options window, then navigate through the tree on the left to Environment > Keyboard. Once you've got the Keyboard Settings showing, type Edit.ValidateDocument into the "Show commands containing" textbox, change the "Use new shortcut in" drop-down to HTML Editor Design View and then click in "Press shortcut keys" and enter the shortcut key you wish to use. I suggest Shift-Alt-V as it's not assigned by default in Visual Web Developer 2010. Finally click "Assign" and then "OK"