It is your fear itself which will cause your spell to malfunction, and allow spirits to take advantage of you. Some asshole once said, "For pure will, unassuaged of purpose, delivered from the lust of result, is every way perfect." As long as you clear your mind before your ritual, use precise conjurations, and execute them with focus and confidence, you will get the result that you intend.
You can verify a spirit's identity by interacting with it. Look at King Belial's Ars Goetia entry. If you are able to see the spirit, he should appear in the form of two angels sitting on a chariot. Sometimes a spirit may appear in a different form to you though, so either way, you should then continue with an interview. The Ars Goetia states that King Belial was created next after Lucifer. So you can ask the spirit whether he or Lucifer were created first. It also states that "He speaketh with a comely voice, and declareth that he fell first from among the worthier angels". So you should ask it to tell you how it fell and it should declare this. The Ars Goetia also states the Belial will not give a true answer without an offering, so be sure that you offer something to the spirit as soon as it appears.
Even having been given these answers, however, you may continue to doubt. No matter what you experience, there will always be some reason to doubt. How do you know that the physical world is not an illusion? How do you know that you aren't dreaming right now? The fact is, you don't know. No one does. To be successful in magick, you have to be okay with that, and simply allow your experiences to speak for themselves.