WARNING: The following article contains spoilers for Batman #51 by Tom King, Lee Weeks, Elizabeth Breitweiser and Clayton Cowles, in stores now.
In Batman #51, Bruce Wayne faces something that even the world's greatest superhero can't overcome: jury duty. Mr. Wayne was selected to be a juror in a case involving Mr. Freeze and the death of multiple women. For that reason, the faithful Gotham City resident does his civic duty and agrees to be sequestered for the length of the trial. Of course, this means that, no matter how much he wants to, Bruce can't risk sneaking out of his hotel, where he is currently on lockdown, in order to patrol the Gotham rooftops as Batman. Because of this, another member of the bat-family has no choice but to slip into the cape and cowl of the bat -- one who is no stranger to the mantle of the Caped Crusader: Dick Grayson.
In order to keep the secret identity of Batman a secret, and to keep helping the citizens of Gotham, Dick slips back into a costume he has worn a few times before. His first stop? The rooftop of the GCPD, where he finds Commissioner Gordon, waiting by the bright bat-signal. But this is where the Commissioner proves his worth as a policemen. Without saying anything, Jim instantly notices that he's not dealing with the same Batman. He may not know that Bruce Wayne is under the cowl, but he's still able to tell when it's not the same person.
Before Dick, as Batman, can say anything to Gordon, the Commissioner speaks first: "You're not him," he says. "You're that other him."
This isn't the first time Jim Gordon has fought besides Dick Grayson's Batman, of course. In fact, the two worked together to save Gotham multiple times during 2009's Batman: Reborn era, when Bruce Wayne was believed dead following the Final Crisis event. But the fact that Gordon can instantly tell the difference between the two before Dick even speaks is telling of his relationship with Wayne's Batman.
During the Batman: Reborn days, Gordon realized he was working with a Batman replacement, and the two built a strong foundation of trust. That he instantly knew that this was that Batman, that Gordon is instantly able to recognize Dick Grayson's traits under the cowl, is a nice nod to continuity. What's more, the fact that he follows his open observation up by asking if the "real" Batman is okay speaks to the bond of friendship between Wayne and Gordon.
This also reminds us that, as much as the two would like to believe that no one can tell the difference, there are actually distinct differences between Bruce and Dick's Caped Crusaders. The regular Gotham citizen on the street may not be able to tell the difference, but Gordon can. He's been working alongside Batman for decades, and he's always been a good police officer and detective. He's no fool.
Fans have debated for years whether Commissioner Gordon actually knows that Bruce Wayne is Batman, with the prevailing theory that Gordon could find out if he really chose to. The truth is, Gordon would rather not know because he understands the symbol that is Batman. He knows that he's an important fixture of Gotham, and he doesn't want to unravel the myth behind it. But at the end of the day, the Commissioner can't be fooled -- he recognizes his allies, no matter which costume they wear, and if he's able to distinguish between Dick and Bruce in the costume from behind on a dimly lit rooftop, there's no way he doesn't know Batman is Bruce Wayne.