Judging car performance based on testing can be dangerous, given the number of variables in play. The different fuel loads, tyre compounds and run programmes means even to the trained eye it's only possible to pick some pointers towards where things really stand.
But that doesn't mean you learn nothing. Ferrari is clearly impressive and McLaren-Honda is clearly struggling yet again. By looking at the lap times, spending time watching the cars trackside and tuning in to paddock chatter, you can get a feel of where things stand.
Talking of watching trackside, it surprises me that this is something team personnel don't do more often. For example, I was watching at Turn 10 on Wednesday and on every lap Romain Grosjean was struggling with the rear on turn-in, and probably complaining about it to the Haas team. If someone had been watching they could have told him he was braking about two car lengths later than anyone else. Things like that matter.