Riot has access to a lot of data on your account - data that can conclusively determine what human person (i.e. what individual) is sitting at the keyboard and mouse playing a particular game. They use that data to flag accounts for boosting bans if they are boosting at any point during the season, for any number of games. That being said, the bans are mostly automated and so there is a (very) small chance for an error. If you truly did not boost, you can request a review through Riot Support where a real human will review the games that were flagged for boosting and determine if you were boosting or not. If Support makes the determination that the data says you boosted, there is generally no further appeal. Support agents do not leave bans placed wrongly, and if there is a doubt in their mind as to if you boosted or not they will remove it.
That being said, Riot does not discuss specific data that goes into it as it would allow boosters to avoid detection. There is, however, quite a plethora of data involved that can allow a human to conclusively, with virtually 100% certainty, determine if MMR boosting occurred.