Use your radiographs! Also light touch with the 11/12 explorer, heavy handed touch makes detecting difficult. Time and experience helps a lot so just keep at it and it gets easier. Good luck with school!
Radiographs are good, but not always indicative of calculus. You are so new that you are not really seeing patients with a lot of noticeable calculus. AIR works well to find small amounts. If you feel roughness with explorer and then you scale and nothing is there then it’s probably a rough surface of tooth. You will get better as you experience more involved patients. Good luck!!
NEVER forget your explorer! 11/12 is your best friend! Encourages you to THINK, use a light touch and not scale everything in lieu of a close look.