[Route("check/{usernameOrEmail}")]
[ResponseType(typeof(bool))]
public IHttpActionResult GetUserExists(string usernameOrEmail)
{
    var user = db.Users.FirstOrDefault(u => u.Name == usernameOrEmail) ??
        db.Users.FirstOrDefault(u => u.Email == usernameOrEmail);
    if (user == null)
    {
        return Ok(false);
    }
    return Ok(true);
}

var url = "api/users/check/" + usernameOrEmail;
$.get(url, function (data) {
    ...//Do stuff
});

@ encoded -> %40