如何在结构复杂的方法上编写单元测试？

I'm new to writing tests, so I have a UserService that looks like this:

func (userService *UserService) Authenticate(email string, passwordAttempt string) (*Login, bool) {

 ...
 return login, true
}

How would I go about writing a unit test for this function?

Do I mock my userService somehow?

type UserService struct {
    deps *ServiceDeps
}

type ServiceDeps struct {
    db *gorm.DB
}

The examples I've seen are for just functions, not when it is run on a struct that has other dependencies.

How would I go about writing a unit test?