This is my big issue too. There are some times where I think I have SSO for something, but don’t remember with who, so I spend 5 minutes logging into various providers and hunting down where they say what sites I’m using SSO with, so I can see what to use.

I started adding them to my password manager as a quick reference, but the irony is that this makes the SSO slower than a typical standard login. I almost never use these SSO options anymore as a result.