Update: as of Android 10, File Based Encryption [is mandatory](https://source.android.com/security/encryption/file-based), so any Android 10 device should be relatively safe when turned off. Well, as long as it's using a password, four digit PINs can be easily brute-forced using the right tools.

You can verify if FBE is active by restarting the phone. If it asks for your password and no apps are loaded except phone functions, keyboard (with the default skin) and settings, then FBE is active. After entering the password, your apps should load and notifications should start working.

The rest of this post applies to Android 5 through 9.

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The best way to tell if your Android device is encrypted (and the encryption is actually in use) is to reboot it and see if it asks for a password/PIN with a prompt on a black screen that says:
>To start Android, enter your PIN

If you're not seeing this prompt, then even though your device may be encrypted, the encryption is pretty much useless. This is because Android's encryption works in two steps:

* your files* get encrypted with a 128-bit master key
* the master key is either encrypted with your password or just stored as is

The lack of prompt means the **master key is not encrypted** (or your device is not encrypted at all) and your data could be read by anyone who manages to boot a recovery image on your device such as TWRP.

To enable the encryption (enabled by default on devices shipped with 5.0+) go to `Settings` > `Security & location` > `Encryption & credentials` > `Encrypt phone`.

But to actually have the master key encrypted and your files inaccessible without a password, you need to go to `Settings` > `Security & location` > `Screen lock` > `PIN` or `Password` and tap `Yes` when it asks if you want to use Secure start-up.

This is how settings look like on Oreo, other versions may be different, but the settings you're looking for should be pretty much the same.


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*7.0+ encrypts files, Android 5.0 - 6.x encrypts partitions [(source)](https://source.android.com/security/encryption/)