How do you know when the MagSafe Charger is fully charged?

To see if the charger has fully charged your iPhone you look at the battery status on the iPhone. This is the MagSafe charger. You do not charge this device. MagSafe Charger - Apple

See the following from How to use your MagSafe Battery Pack - Apple Support

You'll see an amber status light while your MagSafe Battery Pack is charging. When the MagSafe Battery Pack is fully charged, you'll briefly see a green status light.

You're welcome.

Are you referring to the MagSafe Battery pack, not the MagSafe Charger? See this, How to use your MagSafe Battery Pack - Apple Support