Hello Basma,
there are various ways to do muscle testing, and we can explore some in our session, too!
Here is a video that explains the background and two self-testing methods: [https://www.youtube.com/watch?v=I2HXS4lh6i8]
When you do it with clients, you usually test their delta muscle (shoulder muscle), and I'd recommend to have a training in order to make you a confident tester! Here is a more detailed explanation and demonstration with a client: [https://www.youtube.com/watch?v=NN1dctmPYqw]