"Can you see the boy?"
No, that would be like "Are you allowed to see the boy?" (still 'to see' with your eyes, not as 'meet him') or something like that.
From the point of view of Polish, such things like "can see", "can hear" and "can remember" mean exactly the same as simple "see", "hear" and "remember" and you don't translate "can".
I'm not Polish, but the way I gather it is that omitting czy from the beginning of a question is more common and colloquial speech. In formal and/or written Polish, czy is used.
Ukrainian also uses чи (czy) at the beginning of questions sometimes, but it's far less common or required as Polish seems to be.
But "możecie widzieć", although technically it's fine, is not a natural Polish phrase. In most cases, "can you see" means the same as "do you see", and "możecie widzieć" is like "you are allowed to see" or at least "you are able to see". And at least with "you are able to see" is better translated as "możecie zobaczyć".