Not sure you can assume that...  but if they were priced too good to be true, they probably are. 

If you can't tell, call NGK and they will help verify by lot code, etc.

Other than that, Turbodan gave you the best advice.  Buy from a reputable dealer like RMATV or a local store. 

(If you were asking for another brand...  I like the Autolite 4052 to replace the B9ES but it's hard to find and might be discontinued.)

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Myself, I've had great results with the ND (Nippondenso) brand
.... now they just go by the name ... "Denso"
The plug ... W27ES-U corresponds to the NGK B9ES
If you search for them by part number, it's  4046

They are a "U" gap plug, and I actually like them better then the NGK
I swear I can even feel the difference whenever I replace an NGK plug

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Whatever brand, where you buy is still important.  NGK is not the only brand that's counterfeited.

My reason for Autolite is back when I was racing I had several NGK plugs fail very quickly.  Seemed like 1 out of 10.  Got to the point where I'd "pre-run" the NGK plugs for a few minutes, take them out and put them in a box marked "tested good".  That stopped the problem of having a new NGK plug fail half a lap into a race. 

I got some free Autolite plugs at a race to try and also one of the magazines did a spark plug dyno test where all they did was swap plugs and measure HP.  Autolite beat out even the expensive fine-wire electrode plugs in terms of horsepower...  not that there was a ton of difference in HP between plugs but Autolite came out on top.  I've never had an Autolite fail prematurely.

Hate to say that the worst plugs I ever used were Champion.  I got them free from the Champion rep in exchange for running decals...  N2G's which were not cheap to buy back then...  So many of them failed I went to buying my own NGK plugs. (this was before I tried Autolite)

And remember, this was decades ago.  Quality of all brands has likely improved just like it has with oils and pretty much everything else.

Gonna ditch all the ngk plugs and try the Denso's, removing often to check mix and to insure plug isn't failing. 

Resurrected bike now only needs a carb (proper main jet - come on partzilla!!!) and a spark plug. Hopefully graduated cylinder arrives today Amazon Prime so I can validate oil pump output and go for a ride later this week!!!

this group is very cool, I've avoided at least 20 mistakes, hopefully nothing has gotten by, we shall soon see!

Hard to tell if ngk B9es are bogus or not
what's the worst that could happen if I use and they are?

Absolute worst?  The insulator comes apart and beats around inside the combustion chamber until it either pounds the plug gap closed or pinches the ring land down and you lose compression.

I like the NGK EG plugs.  Longer lasting than the ES and more resistant to fouling.  Not as expensive as the Iridium.  NGK has been flawless for me over the last few decades.  I run them in my MX bikes and everything else. 

My worst case is seeing another thread from you named "life after meltdown part 2".

For a $3 part maybe it's not worth taking a minute to research the NGK guides and contact NGK if you have any concerns about authenticity of the plugs you bought?

But risking an engine certainly should be worth buying a $3 plug at a reputable dealer you can trust instead of ebay or Amazon?

I'm guessing they counterfeit about every brand...  and when I heard Autolite might be discontinuing the non-resistor plugs I love I bought a bunch of the equivalent of B8ES, B9ES, and B10ES from an Autolite distributor that gave me a great price on some old stock they had...  so now I have enough Autolite plugs for my older bikes to last a lifetime.

Autolite still makes the "resistor" versions... but I don't want to swap out all my plug caps for non-resistor (or convert them to non-resistor) which would probably have been smarter.  And except I have some oddball bikes that have lower (1000 instead of 5000 ohm) plug caps or call for no resistor in the cap or plug at all. 

Same thing is happening with NGK...  I think they are discontinuing the non-resistor plugs... 

That would suck, thanks for the info 

this counterfeit thing is new to this 55 yr old who's replaced 100s of plugs since he was 14 and never experienced one disintegrating inside an engine!  But funny enough, back when my four boys were boys and not men I had Chevy suburban they blew thru front hub assemblies because of counterfeit timkens. 

Do you know the "EG" version of the B9ES is?  And is this readily available, meaning the Chinese haven't tainted the supply chain with their junk?

thanks for the info, really appappreciated 

ps. I did try to find the autolites that another gentleman recommended but couldn't find 

No sequel Mark!!!

Sorry for all the back and forth

Where does one buy to avoid counterfeit?

Interesting reading your rebuild, as I'm currently doing the same, all be it not as quick as yourself, well done  .

Can I ask, did you use any gasket compound with your gaskets, eg blue hylomer?
Did you replace the clutch bearing in the case or any of the others?

Good idea with the pressure test, never even thought of that, currently working my way through this excellent site to read any more engine rebuilds, sure gives me a piece of mind knowing there's plenty of help and info out there !