While in my workbench there is all sort of electrical parameters measuring equipment, my suggestion today this is for consumers them not necessarily be specialist in electronics.
The most practical method to identify a Fake charger, this is by measuring it weight over a small scale.
You may get the actual product weigh information by visiting for example:
Samsung website (Original Manufacturer).
By the numbers, most to all low priced goods on eBay them are Fake.
For example an eBay seller trading Fake chargers he will never write product weigh in his own listing.
But an honest Asian seller, he will offer product weight information too, and he does that so to earn further credibility..
Explanation of what a Fake charger is all about.1) Internal components are fewer and low quality.
2) Electrical input this does not qualify to any Safety standards = unsafe for all people.
3) Expected performance specified by the labeling, this is down to 1/4
Now you may use the above tip even about shopping unbranded charger.
But
only if product weight it is identical to branded solution ( Then this charger came from the Asian factory which produces and the branded one.)
Usually such a product offering will be priced 50% lower than the branded (retail price in your country), but Never - Ever
lower than that.
Pictures below of genuine goods.