If you want to check while typing, I think you are stuck with using Javascript on the field that you need to check. Since Odoo is webbased and web forms are static, input will not be checked whilst typing.

If you want to check afterwards, then simply add the on_change method for the field (the way to do this varies between Odoo v.7 and v.8), where you would check with a method if the input contains any illegal characters.

Hope this helps you out.