For me its the inability to "finish" the caliber of fighter that he ordinarily would have easily dispatched while at his best. That's usually the first signs of grey hair. Larry Holmes started having these types of performances around 1983. As for the other stuff you listed, I don't think its consistent as to which comes first. Some guys start getting slow years before they're chin fails them, while others like Chad Dawson become fragile overnight.
You've nailed it. Holmes definitely was n't the same after '82. Similar to Muhammad Ali post 1975. The likes of Alfredo Evangelista and Leon Spinks would never have lasted the distance had Ali been two or three years younger.
Watch the way he throws his right hand. he'll lose the snap on it and start waving the punch because his legs aren't sharp. You can bs your way with a left hook, if you know how to fight and position yourself, but the right hand gives it away.
The first signs are usually when you see their activity level decrease. For example, when they start taking shortcuts, like staying defensive and coming alive the last minute of each round, throwing fewer combos than usual, or staying more flatfooted than they usually are. They usually try to mask it by saying it's part of the plan, but when you start seeing it frequently "part of the plan", you know that it's really age setting in.
I do not think there is one universal sign. For example Hagler lost much of his reflexes post Sibson but kept the punch resistance, Herol Graham kept his reflexes, but the punch resistance was what cost him in the end.
Inability to `get off`. Over the years, you see so many fighters on the other side of the curve. During a fight either following their opponent around the ring and not getting off their shots. Or getting beat to the punch. They can see the opening, yet can`t let their shots go. But they hesitate that split second to shoot the shot, and the opening`s gone... Speed and reflexes...
See James Toney, slurred speech, hanging around fighting Tomato cans, trying to make a few bucks, when you should have hung up your gloves, 7-8 years ago and , took a job in the private sector..