Can/Could in the sentence
"Can" is the present tense and "could" is the past tense. If you are currently sharing your screen you ask "Can you see my screen?" but if you have stopped sharing your screen you say "Could you see my screen?".
This applies even if you have stopped sharing your screen only a few seconds ago, no matter how soon after closing it you ask "Can you see my screen?" the answer will be "No" or "Not any more"
However if you are asking another person to share their screen you may say either "Can I see your screen please?" or "Could I see your screen please?" Both are acceptable but 'could' might be considered very slightly more polite.