I have two rule-of-thumb tests to measure this. They apply to any job, at any level.
The first is concerned with pride. Are you, at the end of the day, or the week, able to be proud of the job you've done - confident you've used hard-won skills to do honest and effective work that's made life a bit better for your clients or customers? It's a suitable test for everybody - doctor, waitress, office cleaner, plumber, city broker, chief education officer, teacher.
And second, do you go to bed feeling relatively comfortable about the thought of work the next day? Everyone gets the Sunday evening glums, followed by the Monday morning blues, but the yardstick I use here is of an uncle of mine in Barnsley who was, I now realise, being bullied by his boss. I have the most vivid childhood memory of him saying in distressed tones to my father, his older brother, "Fred, I fair dread the morning."
Do you? Some teachers do. I know one who would drag out Sunday evening, and then read in bed and try to stay awake so strong was that dread.
So - you feel ineffective, and you hate going to school. Does that mean it's time to leave? Well, it might. And you'd do well to take seriously those older colleagues who, beset by mortgages and family commitments, can't understand why unhappy younger ones, relatively footloose, don't just cut and run. It really does get more difficult the longer you leave it.
Even so, it doesn't do to be too hasty. It's possible that you have the ability and the personality to succeed, but not in the school or the sector you're in at present. Maybe it's a particular person, or a set of short-lived circumstances (bad management, Ofsted panic, a school that's generally failing).
If that is the case, it makes sense either to hang on a bit and see what happens, or try again somewhere else. It's not difficult to move - there's an acute teacher shortage, remember. Just knowing that you could do it is a comfort in itself. But choose carefully next time.
Visit, ask, compare, discuss, think.
You're probably not going to give yourself a third chance.