Battery from AliExpress for an electric bike. How to check if the capacity is real before buying. 

To recognize deception and fake capacity, an ordinary calculator is enough. There is no need to read good reviews and rejoice at a successful find. You need to count the ampere-hours.
Dishonest sellers literally flooded Aliexpress with batteries of unrealistic capacity. This is calculated at a time, even before the purchase. But even if the calculator check is passed , there is still a risk of getting batteries with sand, instead of filling and with a much smaller capacity.
Therefore, it is better to be safe and give preference to sellers with the "reliable brand" or "top brand" icon ("Top Brands" in the mobile version).