How to mock an internal call when writing UI test via espresso for android

I am currently writing a UI test with espresso for an android activity. How to mock the result variable in the below code?

As you can see the

Such that I can test the flow for the UI without calling the checkUser() method because it gonna connects to the internet. I am using MockK, but other mock frameworks are fine as well.

Thanks

package nz.salect.handset

import android.os.Bundle
import nz.salect.lib.auth.Auth

class HomeActivity : SalectActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_home)

        val result = Auth().checkUser()
        if (result.error) {
            showError()
        } else {
           // do something else
        }

    }
}