For this example I am going to use some well know Scam batteries that you can buy off places like eBay or Amazon. Doing a quick search on eBay for NiMh and you will likely find something like this:
So lets get back to the point of this post. You bought these cheep batteries (hopefully you didn't because you read this post) and you want to know if they are any good. The easiest way to test them is to pick up a good, trusted, legitimate rechargeable battery and then pick up one of these. Which one is lighter? Which ever one is has the lower capacity (generally speaking). Now we will do the same thing but this time with the help of a scale.
First up eneloop:
Next a Rechargeable Rayovac:
Okay what about a non low self discharge battery from Harbor Freight?
Now lets weigh the scam batteries!
So what is inside of them? I pulled one apart for you and found just a wad of paper with chemical on it (the picture is only when I started, I forgot to take a photo when it was all the way apart).