Unfortunately, this particular USB external battery pack does not provide such information at all. This is so called "dumb power supply".
If you need a "smart one", then you are looking into something different. It's usually something called UPS (Uninterruptible Power Supply) with USB interface. This way you can install NUT (Network UPS Tools) and be able to monitor your UPS from command line using
upsc command. You can check the current battery status, power, remaining time, etc.
However, please note that not every UPS device supported by NUT. Make sure you are getting a compatible one if you decide to go UPS way.
Also, there are some UPS devices specifically targeted to work with Raspberry Pi. They may be able to provide the information you need and possibly are more compact than usual UPS devices.