How do I see what streaming accounts are already tied to my PS4?

Yes, there is a way to see all accounts for services that you've linked to your PSN account. Each user on the console is able to link accounts to services as they choose, so you aren't limited to one account per console per service. However, each user can only see their own account links. To see these linked accounts, here are some directions.

From your home screen, go to your Settings:

Then, go to "PlayStation Network/Account Management":

From there, go to "Link with Other Services":

Once there, you'll see a list of a bunch of services you either have or are able to link with your PSN account. The ones you have linked are marked with a check mark:

If you click on a service that you've linked to your PSN account, it will display the account you've linked for that service. For example, YouTube will tell you the name and email of the account you've linked to your PSN account.