How would I know if I am capable enough to do a phD in physics?

I am currently an undergraduate student, have published 2 research papers under the supervision of my mentoring professors, have a 3.9 cGPA but still haven't taken the GRE. Despite all these accomplishments, I've heard from various clmates and professors that success in undergrad is indicative at all of success in grad-school. Basically, according to them, any idiot can get a 4.0 if they work hard enough in a physics major. So, is there any way to know that I meet the intellectual cut-off to complete a phD in physics?