it isn't hard to get a copy of "game film" even before the formal exchange between teams.
You know teams analyze that for tendencies, down and distance as well as what plays are run from various formations.
You know the analysts will separate the film so a DB can look at a WR and what they do on every play. Their moves, their routes, etc. Non of this is illegal.
Same would apply to the OL looking at the moves used by the DL and they type of blitzes they have used in the past. You can draw conclusions from that. In some cases and opponent may tip a play themselves. how their hand is positioned, how far back do they line up from the LOS, etc.
to say that sign stealing is the definitive reason for a team being better than another, is hard to believe. And that is before you start to account for the skill, experience, speed and/or strength differences between players.
As Prime said, I can give you the playbook, but that doesn't mean you'll stop me.