According to Osho, you can never be, nor need be, absolutely sure about a guru. Be absolutely sure about yourself and your search; from that inner certainty, total surrender becomes possible. A true master remains paradoxical and supra-rational until you yourself awaken, and may even cultivate uncertainty to prevent a mere bargain. Transformation happens when, despite doubt, you risk commitment from your own clarity, not from guaranteed proof.
Don’t wait to be 100% sure about a teacher; be sure about your own longing for truth and take a brave step even with some doubt.