Common research methods involved measuring reaction time to making choices, etc. A check often made is to have two or more people independently observe and report what they saw, to try to obtain interobserver corroboration.
Nonscientific instruments are often both inaccurate and imprecise, e.g., the gas gage on a car.
Valid measures should correlate with other measures of the same concept and with outcomes expected of the concept. E.g., how long one can balance a ball on one's nose will not correlate with other measures of intelligence, nor predict success in school, etc. Reliability is the consistency of a measure. Hypotheses must be stated in such a way to demand verification by examining evidence beyond the definitions of the terms. Simple explanations are appealing and desirable, but one must guard against oversimplification and overgeneralization by avoiding explanations that go beyond existing data and established theories.