You don't. You die with it. XD Jokes aside... To me, I think being able to draw anything with structure from your imagination with a fluent process is good enough. You gotta have some things you like right? Can you draw that from your imagination as of its structure? In different angles? Or if you are more painterly person, can you paint those shapes how you like it to be lit and in any directions? If you can I think you are good to go. You don't need to be perfect or even good in that... It's just you 1) know the structure, 2) know your basic process so that 3) with some proper reference you can then draw that stuff you want pretty much dead on (in terms of that "visual sense", doesn't mean absolute likeness, or else no one can do that XD).

Maybe, it is not that hard. Maybe we would know when we are ready to stop studying perspective, just to name one of the fundamentals, when we know how to solve anything that we want to solve, and we are focus on the design instead of wondering if we would draw good enough a given thing. Of course, we need to try to draw something a little bit harder than before every time. There is no gain in trying to draw a very complex scene to see if we can, because when we are starting, there is no doubt that we would not, and if we believe that we can, is because we are not even ready to see our own mistakes. That is my opinion, hope it helps.

I think sometimes, if you've spent time practicing the basics, it might be a good idea to try moving on even if you don't feel you're completely "ready". After all, no one's ever 100% ready, and you can always go back to review the basics if you feel out of your depth! Even with anatomy and more advanced topics, they're something you'll likely go back to again and again to refresh yourself, or understand something you never got the first time. Challenging yourself is a good way to continue to grow, and I find that learning new info often improves my understanding of the basics as well.

Guys, im just started studying leg anatomy and need good free reference picture

There is really a yin and a yang to all this. The answer for me is that you're never really done studying the fundamentals, but that shouldn't stop you from making the best art you can. Andrew Loomis talked about setting aside one day for study, and I know Norman Rockwell took classes all his life. I think they recognized that mastery is illusive, and there is always a deeper understanding one can achieve. That said, I think there can be a tendency to put off tackling big art projects because we don't feel ready. That's paralysis of analysis. People who do nothing but exercises, but no finished art fall into this trap. Sure, it might not be perfect, but so what? Dive in. If it doesn't turn out great, then it was just a draft. Furthermore, great art can survive mistakes. We're living in a media culture largely built on the shoulders of Jack Kirby comics with all of their made up anatomy. Art doesn't have to be perfect to be good.