How to identify potentially unsafe electrical connectors in use?
Buy from reputable firms

Or to be more precise, don't buy from firms who have workers with apartments in Shenzhen or Hong Kong, because they spend so much time there turning over rocks looking for super cheap suppliers of goods to private-label. That means you can cross off the list:


eBay
Amazon Marketplace, and possibly "sold by and ships from Amazon" as well
Alibaba/AliExpress, obviously
The orange Borg
The blue Borg
The green Borg "11% off"


And we're down to pretty much Grainger, McMaster-Carr, maybe Fastenal though they have some dodgy stuff, and of course, your friendly neighborhood local electrical supply house. 

It's the same rule as auto parts.  I buy my auto parts at shops which have a fleet of small pickup trucks because they do 90% of their business out the back door to mechanics.  Why?  Mechanics have to stand behind the work, it's their time, rep and loaner cars at stake.  A high failure rate will put them out of business.  It'll put me at the side of the road, so our goals align. 

Oh, but price!!! 

Electrical parts have a very low cost to weight ratio, and that makes them expensive to ship.  Another example of such a product is bottled or canned sodas.  For those, the distribution model of "mail order to the consumer" makes no financial sense, it makes sense to distribute bulk materials by truck to local outlets and sell retail.  You see that in prices, like $180 total to ship $105 of wire, or a 75 cent item at the electrical supply costing 75 cents "plus $6 shipping". 

"But I have Amazon Prime! Shipping is freeeee!"  

There's no free lunch.  You pay for the Amazon Prime shipping one way or the other, and the $99 membership fee is not the way. They overcharge for the item and lowball or zero the shipping.  Now that 75 cent item is $3 -- and if you go check out the fees Amazon charges its Marketplace partners to use Amazon fulfillment,  you quickly see why it's $3.   

So to be competitive, there is huge motivation to use counterfeits.  And absolutely nothing prevents a Marketplace seller from selecting a genuine Wago SKU and claiming to have that exact item when it's actually a counterfeit, or right off the Wago assembly line that are either dumpster-dived QA rejects, or stolen good product.