Technically, there are no "certified" CPAP machines, at least as that term is defined at law.
The Air Carrier Access Act does give you the right to use your CPAP, but only if that CPAP has undergone rigorous testing approved by the FAA.
Carriers, except for on-demand air taxi operators, who conduct passenger services must allow, on all aircraft with a capacity of more than 19 seats, any passenger with a disability to use a ventilator, respirator, CPAP machine, or an FAA-approved POC, unless either the device does not meet FAA requirements for medical portable electronic devices and does not display a manufacturer’s label that indicates the device meets those FAA requirements or the device cannot be stowed and used in the passenger cabin consistent with TSA, FAA, and PHMSA regulations.
emphasis mine.
The testing is cost prohibitive, hence none are certified and none bear the required label.
Read more about the ACAA
here.
Some airlines maintain their own list, but that list is not ubiquitous across all carriers.
RE: Delta. I witnessed a man using a Everest 3 on a fairly recent NRT-JFK, so perhaps there is some leeway?