If you cannot reliably land a vital skillshot on a champion
Reliable meaning you can land the skillshot at least 50% of the time
For example,
W
E
E
Q
Do not
Go into ranked
Claim to be good at the champion
Engage in risky fights
Pass go
Why? On a champion such as Vel'Koz it is nearly mandatory to be able to reliably land his E. Without landing his E it is hard to lane one, let along both Ws. Time and time again I have seen people (including me) go into a ranked game thinking that they are good at a champion after a few good games then fail to land a full combo, deal little to no damage, and lose. EDIT: K fine. I changed it.
's W may not be a skillshot, but don't doubt me when I say I have to chase people across the map just to heal them. 
Qs. You just might not like who I end up grabbing

Where are your skillshots now?
Right up your nostrils, wind man.
ult, you are better off not playing Sona. But if you can't land
's Q, you will still be a 100% functioning ADC.
who cannot land a Q if it meant saving their soul