Impostor Syndrome is, or has, become like Asperger's, where every dev thinks they have it. It's a "humblebrag". I honestly tell people I mentor, be confident, but humble. You have this job because you're qualified (or you made yourself qualified), so have confidence in your skills - sit up right, answer questions, take ownership. But also be humble. Don't be afraid to say I don't know, admit you don't know everything about developing.
It's OK To say "I don't know, but I'm doing my best, and if I did, I could answer you."