Ask HN: Can you detect a drive-by zero day browser exploit?

Say you visit a page that has a zero day vulnerability on it, is it possible to tell if your computer is being compromised, such as browser freezing, unusual disk activity, crashing browser etc.? If it is detectable, is it possible to stop it midway through it (possibly downloading and executing)? How did the people at CanSecWest come up with so many zero day vulnerabilities? Does it take a genius to discover such vulnerabilities? What tools are used? Is it a huge leap for a software developer to become a penetration tester/security expert?