How do I know if the Apple Pencil is turned on

Your pencil should always be "on". there is not off or on switch. If your pencil is not working, then it needs to be charged.

Attach pencil to your device.

It will tell you to enable bluetooth if this is not on.

It will show you the battery percentage.

The Apple pencil 1 needs to be plugged into the device to charge. The Apple Pencil 2 attaches to the iPad.

After pairing, the Apple Pencil is always turned-on.

When idle, unmoving, the Pencil will switch to a very low power state with its Bluetooth (BT) radio turned off; as soon as the built-in accelerometer (movement sensor) detects any motion, all its electronics including the BT radio switch-on and communicate with the iPad.

What is crucial is keeping the battery charged and healthy - so as to protect the battery from premature death. A first generation Pencil must be closely monitored to ensure that it is kept charged. A second generation Pencil is easier to maintain, as the Pencil should be kept docked to the iPad when not in use; this will keep the battery healthy and charged ready for use.

The biggest risk to the health of the battery is for the Pencil to be set aside, unused, for an extended period. Even in its idle “low power” state, the battery will eventually deplete. If allowed to fully discharge, the battery may be irrecoverably damaged.