ALS occurs in one limb initially, almost always a hand or a foot. First signs are clumsiness, then clinical weakness, then fasciculations specifically caused by neurones dying. By the time the fascics kick in you'll have obvious clinical weakness. Once those neurones are dead they're not resurrecting, that control of that part of the muscle is gone. You'd very much know.
Aside from that, it would be very atypical to occur in a bicep and thigh simultaneously. It would be very atypical to occur below the age of 45.
If you want to look up something, look up BFS. Though in my opinion this is a fancy way of saying "neurological somatic manifestations of anxiety" anyway.