There is a light on the side of your jamstik(1st Generation WIFI) with the charging port with a small battery icon over it. When charging with the USB cable, The light will turn solid green when the Jamstik is fully charged, while remaining red until then.
Also, In the App Jamstik Connect - Go to the, "My Jamstik" Tab, and when your jamstik is connected, there is a battery icon which will display remaining power.
If your jamstik power light starts blinking red and green, you'll know you have very low battery.
If you are wondering how to charge your jamstik, please refer to the article about, "charging jamstik"