How can I test that a lambda is returned from a function in Kotlin

I have a function that updates a ViewModel Livedata with an Event which contains a lambda. () -> Unit and I want to test that my lambda is returned in my LiveData. With an object was easily done with an Assert.equals but now with a lambda I have no idea how to do it.

Here's what I got until now.

fun retrieveData() : {
  viewModelScope.launch{
   val myData = usecase.retrieveData()
   if (myData != null) myDataLiveData.value = myData
   else errorLiveData.value = Event { return@MyViewModel.retrieveData() }
  }
}

In my test I have:

    subject.errorLiveData.observeForTesting {
        assert(subject.errorLiveData.value!!.peekContent() != null) // This "works" but shows a hint that the comparison is always true even if I try it with == null and the assertion fails
    }

Here's also the Event class.

/**
 * Used as a wrapper for data that is exposed via a LiveData that represents an event.
 */
open class Event<out T>(private val content: T) {

    var hasBeenHandled = false
        private set // Allow external read but not write

    /**
     * Returns the content and prevents its use again.
     */
    fun getContentIfNotHandled(): T? {
        return if (hasBeenHandled) {
            null
        } else {
            hasBeenHandled = true
            content
        }
    }

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peekContent(): T = content
}

I take it from here: https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150

Thanks