There are a couple of different ways to check your PSN region. For instance, you could log into the PlayStation website, click your avatar icon in the top right, select Account Settings, then click the Edit button next to Residential Address to see the address you signed up with.
The easier option for most people, though, is to simply enter your PSN ID into PSNprofiles.com. There’s no login required, and it’ll also let you enter another player’s username to see which region they’re from if you’d like.