Muscle testing is very simple and easy to do, once you learn how (although mastery of the technique requires attention and focus).
A simple method is to make a round ring with your thumb and middle finger (of the same hand). A strong result means that with some tension, the ring won't break, while with a weak response, the ring will break.
When practicing, you can ask your field to give you a 'yes' response, then ask your field for a 'no response. See if you can feel the difference. It should come with some practice.
Note: if you get strong responses for both 'yes' and 'no', or if you get weak responses for both, or reversed responses (weak for 'yes' and strong for 'no'), it means the technique is not working right. One thing that can cause this is weakness in your energy field, especially the root chakra, aka base chakra (which I called the Third Energy Center in my book "The Twelve Human Energy Centers, Exploring the Blueprint of your Being.")
If you're new to the technique, or have some experience, let me know what you find! All the best!