Tough question to answer, but in my opinion, we must look at our results. Everyone will tell you not to be results oriented in poker, but over time, if you are spending more than you are winning, than you are probably an average poker player.
One of the things that makes it difficult to measure, is that as we begin to win, we might move up in stakes, and accept that losing is OK now because we are playing tougher opponents. This is fine, but then we may not even be average now - we are probably a below average player. This all depends on our competition.
In live play I would cash 23% of the time, and I convinced myself i was better than average, since only 10% of players cash in any particular tournament. If this were true, then I should be making a profit, but I was barely breaking even, or losing. So, I thought I was good, but can't string together a bunch of big wins. Maybe not as good as I thought I was, eh?
So - keep studying, keep learning, keep adjusting and find the games you are good at, and play them.