Best way to know is to look at the roots (but be careful when you slide it out of its container). Second best way, if you want to be more sure before messing with the plant at all, to know is to look for roots poking through drain holes. If you don't let it sit in runoff, when it's ready you'll see several root tips just poking out.
I go from a 2" seedling pot to a 1 gallon plastic pot to a 5 gallon fabric finishing pot. It usually takes between 1 and 2 weeks to grow out of the 2" or 1 gallon pots.