Yes thats true, there's no rigorous evidence-based test for consciousness. For e.g there is no test available or just not possible or yet possible or could be possible for e.g a doctor to tell how a patient feel/experience from his report, blood or any test. He has to ask the patient and then prescribe him accordingly. Which is all about subjective experience.
All five sense organ project their sense on consciousness screen. For e.g vision is projected on consciousness which seems to be inaccessible.