It's garnered from watching him and getting a sense of him from the sidelines. He's incredibly dour, and I've witnessed players look visibly pissed off/bitter towards him compared to their reactions to the likes of Vieira, Wilcox and Carlsey. It is notably different. Heads down, shaking and dejected body language. Very little motivation from the sidelines. Stands there silently with his arms crossed and only really pipes up when there's sumat to moan about.
Also for others, and I can relate to this too to an extent, it's garnered from talking to lots of people with very close connections to the players. Family, friends etc. Heard that a lot of the players don't like him, and that's not an uncommon opinion either.
If the players aren't motivated by him then it's futile. In my personal case it's not about results, its about the reaction to him from the players and my personal overall impressions too. You can read a lot from body language, what people do and also, quite literally, what you're told.