If you trust the tech
- Android will say in the settings that encryption is there from the first start on.
- With a screen lock, TWRP would prompt you for unlocking and only then would be able to access the data partition.
If you don’t trust the tech, I guess you would need to copy the raw partition data to a computer and see whether you somehow get anything useful out of it.
Interpreting what I read here, decryption would even need a component in the phone, just knowing the unlock method and having the encrypted data somewhere else would not be enough … but perhaps I interpret wrong …
https://community.e.foundation/t/is-my-phone-encrypted/22779
… else the phone would shut down in 30 seconds?
The screen related to unlocking would just annoy you with a message and continue booting after a few seconds without any user interaction.
If you have the screen where you really have to press the power button to continue booting, the system didn’t like something you did, but you can get rid of that …
… works once Android or TWRP are booted.
No, I don’t know about that. Would have to try that myself.