I am a male contractor with an apprentice that works under me every day. He has complimented me several times and sometimes asks very personal questions, how do I know if he's interested?

Contextual clues could be very important here. Is he complimentary and personally engaging to others in your workplace, or just you? If he is giving you special consideration, then is he just a generally "flirtatious" and low-personal-boundary guy, or is it specifically inviting a more "intimate" work relationship? This also depends on what YOU want. If you're not interested, then a statement such as, "I'm glad I'm able to be a mentor to you at work and I hope that our interactions together will help you achieve your career goals" can be a polite, but firm, shut down. If you're interested, I suggest recommending another mentor for him so that you can keep things ethical at work.