How would I know if my son having infantile spasms?

Salam seizure. Infantile spasm is a cluster of subtle movement of upper extremities and head bobbing which can be just for a second and can be back to back as a cluster up to 100 in severe cases, some times the infant can have multiple clusters on one day mostly after waking up or before going to sleep, if you suspect it in your son try to record the event and check with your doctor.
Brief body jerks. Brief body jerks either toward or in extension. You need an EEG to make the diagnosis and differentiate from benign myoclonus.