Can someone tell you're high after a while?
Yes, do not smoke before school or before anything important. Your eyes get bloody and if the person knows you, they will be able to tell.
How often do you need to use it in order for it to change your behavior (memory etc) and your urine just in case?
You prob wont even feel it yourself but it does kill braincells. Making it harder for you to learn things and you'll get a worse memory. I've also heard that after you smoke weed the police can tell for the next couple of months by picking your hair. Im not sure if its true or if its a myth but. Your urine changes quite quickly.
If you're working atm or in school i would dodge weed, since (atleast in sweden) after you've got your first charge you most likely wont ever get a job.
P.S: Finally an intresting post, thanks!