According to RB they're a bit ahead of schedule. Sure you can't see who is fastest from testing, even if the teams are going all out, it's way too easy to sand bag by just taking strange lines through the corners.
But you can still learn stuff. Like Mercedes sector 3 showed they were really fast. The cold temperatures may have been flattering the car as well.
Luckily we have last year's race to compare. We know cars are just evolutions of last year so pace should be similar. So we can roughly see at least a relative performance vs last year. Again, it's too easy to sand bag.
If fastest race lap was in the 18's last year, RB needs to achieve that if they want to truly challenge.