a bad seal on a stopper would not cause oxidation. CO2 is heavier than air and will form a protective blanket on top of your beer that will not be penetrated by the air.
Oxidation does have a visible sign of sorts. It is a picture of a really nervous newbie who keeps playing with his beer until his fidgeting around with it causes air to mix in with the beer. It is rare but much more prevalent among NERVOUS NEWBS.
On Tap: 1. Kelly R. IPA, 2. Roter Hund Hefeweizen, 3. Bud Killer Blonde, 4. Red Dog Pale, 5. Roter Hund Oktoberfest, 6. Pumpkin Ale, 7. McRed's Stout (with new nitro system and stout tap,) Cream Soda, 8. ESB # 3, & 9. Ordinary Bitter.