You ask one question and then wonder about a second question. I understand from reading your post that English may not be your primary language.
The title of your post is "Finishing
ON her mouth", yet in your reply to Brandye, you talk about finishing
IN her mouth. There is a difference.
If you want your semen to fall on her lips you have a couple of options. The first is to warn her verbally or non-verbally that you have climaxed and the semen is about to come out. The second is for her to press her thumb against urethra at the base of the penis in order to feel the ejaculate move.
If you are concerned about oral stimulation then please read the article listed in the Index that addresses these concerns and gives how-to information.
How Do I Get Him/Her to Orgasm From a Hand/Blow Job?