If you know or can see that someone hat something, ask polite question with 'could ', or 'may' .it's also possible to use 'can'
in more informal situations. In the past, 'can' was not used when asking for something, but only to refer to ability. In the UK,
Cambridge University publishes English teaching materials with the phrase 'Can you lend me.......,'Can I have ......,'etc. In the
United States, this form is still considered to be incorrect and 'May I have ....' is preferred.