Sure, but once you see it, the "casting" is done. There's nothing to counterspell any longer. At that point all you are able to do is identify the effect of the magic. Counterspell explicitly interrupts casting and can do nothing once it's done and the effect is taking place.
Edit: The direct applicable rule is of course Counterspell itself. Again, if the effect is happening, it's too late to interrupt the "casting" process of the beholder.
"You attempt to interrupt a creature in the process of casting a spell."