What's the best/restful way to design an API endpoint for checking existence of resources?

For example there is a user database. While new user tries to sign up I want to check if email has been used on-the-fly.

My idea is: `POST /user/exists` and payload would be something like `{"email": "foo@bar.com"}`. The response would be either 200 OK or 409 Conflict.

Is this a proper way?

Thanks!