If you just get a glimpse of someone turning a corner, maybe just a foot, can you cast a spell that says "that you can see"? A situation that came up for me was as follows.
You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn. The spell has no effect if the target is undead, if it doesn't understand your language, or if your command is directly harmful to it.
A gnome is on my back and I want to cast command to get it to leave. I can only see a hand, but he can hear me. Can I cast command on him? What about skeletons just cresting a sand dune? And can I do a melee attack?
What I'm trying to get at is what it takes to see someone. Do you have to see center mass, a whole body, do they have 3/4 cover if you can only see a hand, even though it is not covered, just behind me?
Best Answer
Total cover blocks spells and attacks
PHB 196:
Going off of this, as long as a target isn't behind total cover, you should be able to target them.
If I were the DM at the table where this question came up, I would probably require that the caster be able to see a decent bit of the target, like a hand, head, or foot rather than just an eyelash, but that would be up to each individual DM.
In this case, the gnome is certainly not behind total cover, and should therefore be a possible target.
As long as they aren't behind total cover, you should be able to target them, although they are likely going to benefit from three-quarters cover if you can only see a bit of them.