Markus,
My advice would be to have a copy of your license ready and ask the flight attendant once you board for permission to "view the cockpit." Do not ask in an "aggressive" tone. Be friendly and just ask in an innocent tone. My guess is that the United flight out of Frankfurt will be a 777 - which boards near the middle of the aircraft - you will likely not be able to see the actual cockpit from the entrance. Your flight to Dallas will likely use a 737-300 or a 727 - it will be easier to see the cockpit from the entrance because of its relative location. Just be a "nice guy" and have your license ready in case they ask for it. Of course, it might help if you carry a camera and request to take a picture of the cockpit...
Good luck!