How to check if user is alre!dy logged in?

hello i need to check if user is logged in or display universal login

In most cases, when your application needs to authenticate the user, it will request an authentication to the OIDC provider (Auth0) via an /authorize request (how to actually do this will depend on the SDK and application type used).
If you have “Enable seamless SSO” enabled (or if you have a new tenant, where this option is enabled by default and can’t be turned off), Auth0 will show the login UI only if required (i.e. if the user doesn’t have a session). There might or might not be other prompts as well (like MFA or consent), but if no user interaction is required then the application will get the results immediately.

So, in most cases, applications don’t really check if the user is logged in into the identity provider: they just request an authentication.