‘In my humble opinion’ is a horrible phrase and a dead giveaway. Truly humble people never tell you they’re humble. People like Uriah Heep do that. True humility is a great strength, and its principal characteristic is the subjugation of ego, so the truly humble person has no reason to proclaim his humility because he has no reason to care what you think of him.
It follows, therefore, that a person who needs to tell you that he’s humble is manifesting excessive ego inflated by insecurity. Chances are that he will be obsequious, too, so he’s probably better given a wide berth.
(And I might add as a rider that, in my opinion, there’s nothing wrong with being insecure. It’s how you handle it that counts.)