How do I know if I am getting a real oem cellphone battery or good aft

Motorola (Lenovo) does not sell spare parts to the public, so a original battery will not be an option for you unless you can get one from a repair shop that buys their spare parts directly from Motorola.

When it comes to aftermarket batteries it is usually hit or miss, if there are reviews available, check them out, if not, you will just have to rely on your luck. Try finding a reseller that has good reviews and that preferably has their own branded (own import) batteries. It is usually a safer bet than buying some no-name Chinese battery.

A quick Amazon-search gives you one of many options on the net;https://www.amazon.com/Motorola-Replacem...