Nothing. It's not an exaggeration to say I watch every game. Then I try to piece it together. It's a lot harder in sports like baseball or college football because you have to trust other peoples opinions as it's impossible (I've tried!) to watch every damn baseball game every day or catch every CFB game (fuck that sounds worse than hell). In the NFL it's simple to watch every game. First there aren't that many of them and second they have robust tools to help you do so. It's great! I'm not going to pretend to use advanced math or lean on outside experts if that's what you're asking though.
That makes sense, and I've definitely been trying to do more of that myself this season. I don't watch every game but I did get NFL Rewind for once so I'm watching a lot of teams that I normally wouldn't. I agree that it's the best way to form opinions on how good a team is.
I guess the area where I differ is I don't have my own "mental power rankings" so to speak. I can look at a matchup and predict who will win based on what I know about the teams but I usually don't have much confidence in my pick because I realize that in the grand scheme of things it is incredibly hard to predict that kind of stuff consistently and I'm just some dude watching football. I don't claim to be an expert, but on top of that I don't think anyone really is. That's the beauty of the NFL to me.
I don't really try to classify teams on some scale because most of it just feels so arbitrary and it requires too many assumptions. I guess it's easier in other sports like baseball because you play so many games, but in the NFL it's hard to figure out where every team ranks relative to the others because they don't even play half of the teams in the league during a season, and most of the teams they do play they only see once.
I guess the goal is to work with the best information we have (which for you for example would be watching the games and that's it) but even with that I don't feel very confident in any kind of objective measure of how "good" a team is. There are too many unknowns so I'm usually happy with more subjective remarks and feelings on a team.