How can my app find out which NFC mode is enabled?

No, your app can't really check what mode is currently enabled. The point is that, by default, an Android device will periodically check for all three modes (actually it's a bit more complicated than that: those three modes are themselves split into several parts, each polling for a certain technology). So, while the screen is on, you can expect the Android device to cyclically switch between listening for other readers and polling for different types of tag technologies and peer-to-peer mode devices. This is the case regardless of whether you registered to send a message over peer-to-peer mode, an app is registered for HCE, or your app registered for NFC discovery intents. Even if you disable Android Beam in settings, the device will remain to poll for peer-to-peer mode.

Only if you explicitly disable card emulation and P2P using the reader mode API (available on Android 4.4+), your app can control that only reader/writer mode is active (and it can even control which tag technologies should be polled for).

So your app can know what modes it enabled itself and can do some guessing, e.g.

if the screen is on and reader/writer mode was not explicitly forced using the reader mode API, all three modes are likely to be active.
if the screen is off, only card emulation mode may be active (though this is device dependent and HCE is usually not available).

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