'check it out' or someone out is to investigate or evaluate something or someone; observe carefully. For example, I don't know if you'll like the film; check it out yourself, or That man who's staring is probably just checking us out. [Slang; mid-1900s]

check something out: is used in informal English to inquire about something to find out whether it is true or correct. Example: The police checked out his story. (From Longman Dictionary of English)

you find out information about it to make sure that everything is correct/satisfactory.

"Check it" means inspection, see it for yourself. "Check out" means making payment at the cashier. Check it out is a slang, it means it is good, you will take it, see it for yourself.

It is to go and take a look at something, or 'see for yourself'.