Sometimes you know. Julius Erving announced that the 1986-87 season would be his last, and it became a farewell tour. When he walked off the floor after a playoff loss in Milwaukee on May 3, 1987, all knew they were seeing him in a 76ers uniform for the final time.
Sometimes you have no idea. Mike Schmidt went 0 for 3 in an otherwise forgotten game against the San Francisco Giants on May 17, 1989. Nobody in Veterans Stadium that day, including Schmidt, knew he never would play baseball in Philadelphia again. Schmidt suddenly announced his retirement 12 days later before a game in San Diego.
Sometimes, like today, you suspect but can't be sure. Donovan McNabb (and Brian Dawkins and some others) could very well be playing their last game as Eagles today. The crowd that fills Lincoln Financial Field won't know until later whether it is watching an era end - or merely a disappointing season.
It is even possible the big guy on the sideline - no, not Tra Thomas - will be making his farewell appearance at the Linc today. It is a long shot, but not out of the question that Andy Reid could move on after a decade as head coach of the Eagles - by his choice, by Jeffrey Lurie's choice, or some combination of both.
Charles Barkley. Eric Lindros. Allen Iverson. Pat Burrell. All of them played their final games as Philadelphians under similar circumstances. You knew change was possible, even likely, but you weren't 100 percent certain until later. Only Burrell went out in grand style, clanging a momentous double off the center-field fence to start the World Series-winning rally in his final at-bat as a Phillie.
Whether today's game against the Dallas Cowboys has playoff implications for the Eagles or is rendered moot by earlier events, the air still will be highly charged for the fans. There is no shaking the sense that big changes could be on the way after 10 years of uncommon continuity.
Some fans will welcome any change, just as a break from the same old, same old. Familiarity, in this case, has bred a certain amount of contempt and an awful lot of fatigue.
Some fans will be very upset if McNabb goes and Reid stays. Some will be very upset if either goes. These fans see the Reid-McNabb decade as the most successful sustained period in franchise history. They have a point.
Most fans are somewhere in the middle. They can appreciate Reid's accomplishments in building a team that went to four consecutive NFC championship games and one Super Bowl. They can see that McNabb has been one of the franchise's best players and has created some of the most memorable moments in Eagles history. They know it won't be all that easy to replace either the coach or the quarterback.
It is possible to respect Reid and McNabb and still suspect that something has to change for the Eagles to get back to Super Bowl contention - which is, after all, the only point to all of this.
Barring a combination of unlikely outcomes, the Eagles will miss the playoffs again. That means Reid has not assembled a team and reached the postseason in accordance with his plan since the 2004 Super Bowl season. The only playoff appearance since then, in 2006, was the result of a fluky Jeff Garcia-led late-season win streak.
Four futile seasons: That does not create much hope that 2009 will be a championship kind of season, not without major change.
The day Reid traded out of the first round and took Kevin Kolb with his first draft pick, he sent a message that it was more important to replace McNabb than to add talent that would help the quarterback win. The day he yanked McNabb out of a winnable, three-point game in Baltimore, Reid revealed how close the end of McNabb's tenure really was.
Time will blur the details. McNabb's career here, whether it ends today or in 2013, will be judged by statistics and wins and losses. People will argue whether he wasn't quite good enough to win it all or whether he was fatally hampered by Reid's insistence on running a pass-happy offense without enough first-rate receivers.
The most compelling fact: The only full season McNabb played with a great wide receiver, the Eagles went to the Super Bowl. That should end the argument.
There are several reasons McNabb's time could be up here: his current contract, his desire for a new one, the presence of Kolb, Reid's own desire to try his offense with a different quarterback running it. Mostly there is the simple truth that time passes and things change, in life and in the NFL.
So treat this like goodbye, because there may not be another chance.