Depends on who you talk/listen to, but testing/results universally accepted by everyone as diagnostic for ME - not as far as I know. It's a diagnosis by exclusion thingy, and you don't want it as a diagnosis until everything else has been excluded.
Consensus on supplements? I wouldn't have thought so, people generally make educated guesses based on either test results or symptoms and then throw money at the problem to see what works, most don't, some make people worse,
From personal experience CoQ10, Magnesium (both malate and glycinate) and a good B complex (I use Jarrows B-Right), most, but not all, pwME can tolerate a B complex and find it helpful, at least from what I read.
These probably won't hurt and may help, it seems a lot of people use them.
There are lots of other things that may help, probably won't, but may, I'm far from an expert, as my full unused/unhelpful supplement box will testify.
Start off one thing at a time, low dosage until you're sure you don't have a negative response, keep records, if you don't then you won't be able to tell what helps, what causes problems and what does nothing for you.
No biomarker for ME/CFS yet exists. Like the other poster said, it is a diagnosis based on a person both meeting the symptom criteria for ME/CFS and having excluded other conditions that are known to cause identical/similar symptoms. This usually means seeing various specialists to exclude various rheumatological conditions, neurological conditions, viruses, etc.Is there a gold strandard test to confirm ME/CFS? Also if there a consensus that certain supplements help with this illness? Thanks