So, in no particular order...
1.Genuine belief in magic
A witch needs to believe beyond a doubt that magic will work. Any doubts, no matter how much she wants to think magic works, any doubts will blow her spells out of the water. In my opinion this is best demonstrated when a witch has a crisis. Does she rely on her magic? Does she turn to her spell book to sort things out or does she abandon her "hobby" of witchcraft and look to solve her problems with other means. An accomplished witch looks to utilise magic in all aspects of her life perhaps most of all when the going gets tough. (I do get a little fed up of hearing -I'll get back to practising as a witch when x,y and z are sorted out. A true witch willl be doing more practising than ever if x,y and z need sorting...)
You do need a certain amount of knowledge to make any claim to being an accomplished witch. I'm not a fan of reading through tome after tome of dusty esoteric wafflings but a good understanding of the basics is certainly necessary.
3. Practical Experience
I'd value this above knowledge. A witch who knows it all but practises nothing is a poor witch. I see the term witch more as a descriptor of what a person does than what a person is. It therefore follows that any witch who doesn't bother getting her hands dirty can have all the knowledge in the world but that knowledge isn't translated into ability and therefore into power unless she is doing something to demonstrate it.
An accomplished witch creates her own spells. Full stop. You can go a certain distance using the works of other people but a witch doesn't cross the line from novice to practioner until she has a good enough grasp of how magic works to design and craft her own spells. I'd encourage this from the very beginning to be honest. I have no patience with spell books or following other witches' workings. If you want a spell to work, tailor it to the specific circumstances and write the darn thing yourself.
Arrogance is inevitable in beginning a magic path. Its a heady feeling indeed when those spells start to work. But at the other end of the spectrum away from all the witches just beginning to feel their power are witches who know themselves and their power inside out. True power is not needing to shout about it. A truly, truly powerful witch knows that what she is going to do will work and she doesn't need to convince herself or anybody else of that fact. Watch for the witches who proclaim how powerful they are - chances are they are less powerful than those who choose to keep their power to themselves.
It rather goes without saying that in order to be considered accomplished at anything you need a fair degree of success in it. A witch whose spells don't work isn't really very much of a witch.
7. Confident in outcome
Magic works best when approached with a good deal of confidence. By this I mean a good deal of confidence in magic itself, not an inflated idea of your own personal prowess. An accomplished witch casts not on the off chance but with the absolute certainty that she knows what she is doing will bring about the desire effect. This is probably the most important demarcation between the novice and the proficient - the novice witch hopes that what they are doing will work, the proficient witch knows that it will.