how to compute sha digest of usb stick

[hlyg <hlyg2023@outlook.com>]

i have copied trixie live cd to usb stick,

is it possible to verify usb stick with sha256sum without corresponding 
iso file?

i read sha256sum manual, no option for file size

does live cd in usb stick change after running and using for long time? 
that is, all writes are performed in memory?

Thanks!