Before Sisko realizes the O'Brien in the park is a changeling impostor, he says that the Defiant couldn't have gotten from DS9 to Earth that fast. It has been, at that point, 4 days since the attack on the power grid, and Sisko had already been on Earth for several days prior. That means Sisko believes the Defiant couldn't have gotten to Earth in about a week's time. Yet later on, Sisko calls the station for help, and the Defiant arrives at Earth in under a day.
After the Lakota opens fire on the Defiant, steam begins pouring onto the Defiant bridge. The device releasing the steam is visible.
When the Lakota attacks the Defiant, Kira asks if they can cloak. Worf replies that they would have to lower their shields to cloak, which would leave them vulnerable. Kira has served onboard the Defiant on numerous occasions, including assuming command several times. She is well aware of how the cloaking device works and would not have asked such an obvious question.
After Sisko refuses to go along with his plan to overthrow the Federation government, Admiral Leyton orders Sisko back to Deep Space Nine. Sisko has information regarding a treasonous plot. Reassigning him back to his previous post would do nothing to prevent him from spreading that information.
At the end, Joseph Sisko tells his incoming customers that he recommends the "pasta boudin", but pronounces it "boo-DEEN". A New Orleans native, even in the future, would know that it is pronounced "boo-DAN".
The changeling that is disguised as O'Brien suggests that he would like to go to a bar with Sisko, have a pint, and throw some darts. It has been well established that changelings are not capable of eating or drinking.