Throwing a tight spiral on a deep ball is not particularly about arm strength. The two give aways when it comes to limited arm strength is upper body rotation on deep balls and elevation on intermediate and deep throws.
Jay Cutler and Mike Vick, two guys with arm strength to spare can throw almost perfectly level fast balls 15-20 yards down field, that is because they have a ton of strength behind their throws. If your intermediate passes tend to need to get air under them that is limited arm strength. Drew Brees throws a fantastic football, but he can't zip the ball 20 yards without needing to give it a bit of elevation, the same is true for Philip Rivers and Tony Romo and Peyton Manning.
The other tell-tale sign of poor arm strength is a hurling motion on deep balls, Kyle Orton did that a lot when he went downtown, he would rotate his upper body during the throw and almost throw his shoulder into the action - it generates more power on the ball but it does lose you a lot of accuracy.
Manning doesn't have the laser rocket arm, but he is by no means the weakest QB in the league, the wobbly spiral on his deep passes has more to do with touch and finger placement when uncorking it, which could still be a result of nerve damage and loss of tactile response.