When in doubt repeat a portion of the experiment, as there is no better teacher than finding your own mistake. If you are still unsure, the lab instructor should be able to help, ask him or her. Professors will discuss the important aspects of the lab with students during a prelab preparation session each week prior to that lab. Be on time. This is part of your lab period.
Laboratory is a sensory experience full of interesting surprises and fun.
Being prepared enhances that experience for you and your instructor.