Differential diagnosis
Although central and obstructive sleep apnea have some signs and symptoms in common, others are present in one but absent in another, enabling differential diagnosis as between the two types:Signs and symptoms of sleep apnea generally

Signs:Observed breathing pauses during sleep
High carbon-dioxide saturation of blood, especially just before awakenings during which a patient experiences urgent need to breathe (see "Symptoms" below)
Low oxygen saturation of blood
Heart rate increase (response to both hypercapnia and hypoxemia/hypoxia), unless there also exist problems with the heart muscle itself or the autonomic nervous system severe enough to make this compensatory increase impossibleSymptoms:High frequency of urgent need to breathe upon awakening (symptom created by hypercapnia), especially among subset of awakenings occurring at times other than normal for an individuals sleep schedule and circadian rhythmsSigns and symptoms of central sleep apnea

Signs:Lack of abdominal and thoracic movement for 10 seconds or longer during sleep and coincident with breathing pausesSymptoms:Inability, either complete or without excessive effort, to voluntarily operate diaphragm and other thoracic muscles upon awakeningThe combination of this symptom with a high frequency of urgent need to breathe upon awakening is especially specific in that the co-presence of the latter symptom differentiates central sleep apneas presentation from that of sleep paralysis generally.Signs and symptoms of and conditions associated with obstructive sleep apnea
Signs:Observably ineffective respiratory movements (observable lack of air flow despite observable muscle movements indicating efforts to breathe)
Snoring (high-sensitivity but low-specificity)
Observably dry mouth or throat (high-sensitivity but low-specificity)Symptoms:Sleepiness, fatigue, or tiredness, often rising to the level of excessive daytime sleepiness
Frequent feelings of choking (airway and/or lung compression), as distinguished from mere feeling of suffocation nonspecific with respect to presence/absence of pressure, upon awakeningAssociated conditions:Opioid medication use
Large neck circumference (>16" for females, > 17" for males) (frequent causal factor and possible indirect symptom; see "Obesity" below)
Obesity (frequent causal factor and possible, albeit low-specificity, sign both direct and indirect): Obesity frequently involves accumulation of fat below the chin and around the neck, depressing the trachea when one is in the supine position, and central obesity can, depending on an individuals fat distribution, lead to increased direct pressure on the thoracic cavity and/or compressive anterior (headward) displacement of the abdominal organs, in the second case reducing space for and increasing difficulty of the motion of the diaphragm.
During  a PSG (polysomnography) (a sleep study), a person with sleep apnea shows breathing interruptions followed by drops/reductions in blood oxygen and increases in blood carbon dioxide level. In adults, a pause must last 10 seconds to be scored as an apnea. However, in young children, who normally breathe at a much faster rate than adults, shorter pauses may still be considered apneas. Hypopneas in adults are defined as a 30% reduction in air flow for more than ten seconds, followed by oxygen-saturation declines of at least 3% or 4% per the AASM standards. and/or EEG arousal. The Apnea-Hypopnea Index (AHI) is expressed as the number of apneas or hypopneas per hour of sleep.As noted above, in central sleep apnea, the cessation of airflow is associated with the absence of physical attempts to breathe; specifically, polysomnograms reveal correlation between absence of rib cage and abdominal movements and cessation of airflow at the nose and lips. By contrast, in obstructive sleep apnea, pauses are not correlated with the absence of attempts to breathe and may even be correlated with more effortful breathing in an instinctive attempt to overcome the pressure on the affected persons airway. If the majority of a sleep-apnea patients apneas/hypopneas are central, their condition is classified as central; likewise, if the majority are obstructive, their condition is classified as obstructive.