Short of using professional lab tools you can't really determine if a single lens is performing up to spec or not. If you have real stinker you can usually tell that, but without a calibrated optical bench it is nigh impossible to tell if a lens is slightly better or slightly worse than the manufacturers stated performance.
In the real world what usually happens is that you start out using your theoretical $500 lens. At some point you discover how much that expensive (in your newbie mind) $500 lens lacks in certain areas compared to results other photographers are obtaining with higher end gear. You then go out and buy a mid-grade $1600 lens and when you begin using it you are disappointed with the marginal improvement you see between the $500 and $1600 lens. Sure, your results are a little better with the $1600 lens. But they are not the three times better you were expecting when you spent three times as much on the new lens!
Hopefully at some point you begin to realize that great photos are about a lot more than having the best gear. You can't capture a well lit photo in crappy light. It doesn't matter how sharp a lens is if you can't learn to focus it on the point you wish. That f/2.8 zoom lens doesn't help you find the best aperture and hyper-focal distance needed to maximize resolution and Depth of Field of a landscape vista. Just as your experience allows you to begin to understand what you need to do to capture the shot you envision, your experience should allow you to see if your lens is performing in the ballpark that you expect based on the reviews you read and samples you viewed before buying it. If you are not to the point where you trust your own experience to evaluate the performance of a lens, seek out a more experienced shooter in your area that uses the same system and see if they would give you an opinion on the lens. Photography clubs and groups are more common and popular than ever thanks to the ease of communication made possible by the social media revolution.
I'm a Canon guy and have been for years (decades, really). When I first discovered Bryan Carnathan's lens reviews at The-Digital-Picture, I looked at his reviews of all of the lenses I already owned. As I read the reviews I repeatedly found myself nodding in agreement. The strengths and weaknesses he identified in certain lenses matched my own experience with those same lenses. There was a single lens where my experience and his diverged quite markedly. I did a little investigation, borrowed a copy of the same lens from a friend, and compared it to my own copy which I hadn't used for several years. I discovered that my copy was moderately under performing. I also discovered that my own technique had improved during the interim and I could get better results with that same lens than I had gotten when I first used it! Prior to that experience I had routinely dismissed that lens as being a bad design. I had recommended against buying that lens to other Canon shooters. What I learned is that the next time I have a less than desired experience with a lens, I should probably send it to Canon before the lens is out of warranty instead of putting it back in the box it came in and never using it!
