Focus on the symptoms like pain, or gas, or indigestion, and learn from those symptoms what foods trigger it, and what foods your should limit or eliminate 100%.
Large worms, like tapeworms, can also move like small snakes. Tapeworms can be over 2 meter long, but their bodies are very slim, like a tape, and I doubt that a motion of a Tapeworm could be observed by watching the belly.
Tapeworms are significantly smaller than intestines. It is far more likely to observe intestines move, tha to observe Tapeworm moving.