I think you could probably ask 10 different magicians and get 10 different answers.
Some might say you are good when you can perform a routine with confidence and without error.
Some might say you know you're good when you're getting a good reaction from your audience.
Some might say you have to know X, Y and Z effect/move/etc before you can say you're good.
Some might say you know you're good when you are making a living from performing magic.
I could go on and on. Being "good" will mean something different to everyone. At the end of the day, the opinion that matters is that of your audience at the end of your routine.