You can see the technique or lack thereof, you can see the sloppyness of the basics or if the basics are good, you can see the quality of the crossing or lack thereof, and so on, there's plenty of footage to judge the qualities. I'd seen Fred play a few full games, I've seen James play 2, I don't know how many you feel is needed but with Fred even without a full game it was clear in every video that his touch was very loose, that was just evident and you can see it now when he plays for us, it's how he got cost us the goal vs Wolves and it's why he's so risky as the deepest CM, as touch is erratic, the idea ytou cannot have an opinion unless you've watched 20 full games or something is bizzare.
I never shat on Firmino but sure, I mean in hindsight we should have bought Milner and Henderson as well, hell we should have gotten Liverpool's manager, that was something I think most were in agreement at the time, but what he has managed to do with some players in that squad is what Fergie did with ordinary players in a unit that is brimming with confidence and a complete footballing ideal.