1. Constantly using the excuse "I have a girlfriend" to get out of a situation.
2. Acting excessively gay and/or feminine at all times.
3. Constantly being a huge pussy
4. A strong aroma of Bobo Cheese and asshole emitted from the person's mouth.
When discovered, the accused dad-fucker immediately loses all credibility in any conversation. Any attempt by the accused dad-fucker to offer an opinion or statement can and should be discredited and met with the statement, "shut up, you fuck your dad." Furthermore, when conversation within a group dulls, one can and should immediately offer the exclamation, "You fuck your dad!" Regardless of whether or not this exclamation was funny, all other members of the group should laugh and agree.
And remember, Once a dad-fucker, ALWAYS a dad-fucker.
Non-Dad-Fucker: "No you can't Nate, you fuck your dad"