My first twitching was a popcorn-like feeling and I still get that from time-to-time like right now. Some can be seen and some can't--helps to hold a light to cast the muscle in shadow in order to see the tiny depressions made. But I also get the same activity in the form of thumpers (a quick, sharp twitch of a whole muscle), and vibrations (I know they are fast twitches but I can't begin to see them).
I have heard repeatedly that ALS patients generally are not aware of many of their fascics, while most of us in this group probably would admit that we can sometimes feel each little one.
-Dave