The uppers are generic to most all AR rifles, so markings on it could be wrong, even if one exists. The barrel is the best way to check, it should be marked, probably near the muzzle. That marking on the magazine catch is an aftermarket part that can be easily changed.
You can also take it to a local gun smith or gun store and ask for help. Generally they will be glad to answer your questions.
If you decide you don't like whatever caliber it is in, an easy barrel swap ( <$125) can put it to what you want.
Looks like you now have a nice set up, hope you enjoy it...............whatever caliber it turns out to be (my guess is .300 BLK).
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