How Do You Know Your Skills

White hack hackers do not infiltrate real systems unless performing penetration tests. However, since real world hacking is illegal and they don't participate in this, how do they know their skills are up to snuff if they aren't penetration testers? Have most of them even hacked anything? And how can they possibly, legally learn these skills when real world experience is illegal? How can they know if they are skilled? How can they become penetration testers?