How do i know if i'm the first user id apple for my phone?

Just bought an iphone

I don't really understand your question.

If you bought the phone brand new from Apple or a carrier, then there's no Apple ID associated with that device. If you bought it used, then hopefully the person who sold it to you signed out of his/her account.

There's the page to see if it was activated or not which I guess may be the closest you'd get.

Of course if new, then you'd be the 1st. If not, then you wouldn't.