By comparing it with reality. That philosophy which best describes the nature of reality is the best philosophy. Among all the "philosophies" which I have studied, Rand's is one that most nearly describes reality correctly.
By the way, I put philosophies in quotes, because there really cannot be more than one philosophy, any more than there can be more than one chemistry or physics. There is only one true description of any aspect of reality.
Hank
What did Rand say about reality that's not apparent to most of us?