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?