I need to check if a variable is an object of the User type.
User is my class $user my object
$this->assertInstanceOf($user,User);
It's always a good idea to use ::class wherever you can. If you get used to this standard, you don't have to use FQCNs (fully qualified classnames), or escape backslashes. Also, IDEs provide better functionality if they know that User here is not just a string, but rather a class.
$this->assertInstanceOf(User::class, $user);