How do I know if my macbook republished or not?

Unless you bought it directly from Apple's Refurbished online store, it is not going to be refurbished.

Maybe refreshed or reconditioned by a third party, but not Refurbished like Apple does.

There is no way to determine if it has been reconditioned by a third party unless they tell you however.

You can check the serial number, and if it starts with a "Z" it would not be a refurbished unit by Apple.

But may have been reconditioned by someone else, and the serial number would not have changed in that case.

Use this page: Check Your Service and Support Coverage - Apple Support, but it does not distinguish between new and refurbished Macs. There is no need for that since refurbished Macs sold on Apple's Certified Refurbished Products page are considered functionally equivalent to new ones.

Where did you buy it?

Unless you want to crack it open and try and identify non Apple components, you're success in a positive answer is likely going to be nil.

Why do you even suspect it's not all original?