Hi Alex,
xfa.form.execValidate();
This will check all validations on your form, display the error message for each, and return true or false. True if all validations pass and false otherwise. Use this in your print button to check the validations before printing. So. . .
if (xfa.form.execValidate())
xfa.host.print(yada yada);
A word of caution though. I don't think this is fool-proof. A user can always print using the menu or a shortcut key in Acrobat/Reader, which bypasses your print button.
Hope this helps.
Jared Langdon
www.jlangdon.ca