Is muscle testing legitimate?
This depends upon who is doing it, how they are doing it, and why they are doing it.
What can muscle testing tell you?
You can attempt to ask pretty much anything using muscle testing, however, the more objective the question and the less attached you are towards a specific answer, the more reliable muscle testing will be. Personal biases including attempts to avoid discomfort or seek comfort, pleasure, or gain as the primary goal, attempts to control others, breaches of privacy, etc. can all affect the outcomes of muscle testing.
What are other names for muscle testing?
Applied kinesiology, Muscle Response Testing, Nutrition Response Testing, Emotion Code, Body Code
Is muscle testing spiritual?
Anything and everything can be considered spiritual, depending upon how you choose to view it, including muscle testing.