I wouldn't have thought it should really differ too much from how you'd evaluate normal suctioning - auscultation, palpation, observed work of breathing, pt's reported dyspnoea (if possible), SpO2, may ABGs, weight of sputum (if facilities exist).

One unique thing though is that what you suction up may be black - you could see when the secretions cleared change colour. Are there differences when the patient is in different positions?

Is there anything else?