On the home screen you’ll see a battery indicator that graphically shows your current state of charge. If you press ZL+ZR, you’ll see a battery percentage below it. And in the settings, you can turn on a permanent battery percentage on that screen.
Just check the battery indicator on the home menu. The post above me explains all that fluff. Keep in mind that your joycons charge by being docked to the side of the console. Once the Switch console is fully charged, then only will it start charging the joycons.