Does the player know the audio and video are out of sync?

Generally ExoPlayer runs the playback clock based on timestamps from the audio track, and aims to release video frames to the screen so they are in sync with the corresponding audio timestamp.

If you see A/V out of sync some possible causes are:

1. Video decoding is too slow, so the frames aren't ready when they need to be released to the screen, so they are released late. In this case the player knows it's out of sync. The demo app displays 'video frame processing offset' (vfpo`) in its debug text at the top of the playback screen which measures this. See more info here: https://medium.com/google-exoplayer/improved-rendering-performance-operating-mediacodec-in-asynchronous-mode-and-asynchronous-buffer-3026207850b2
2. The media has incorrect/mismatched timestamps, so the player does everything "right" but the user perceives the audio and video are not in sync.
3. There's a bug in the player logic which is aiming to keep audio and video in sync (and so by definition, the player doesn't know the audio and video are out sync).

In case 1, you can measure vfpo from DecoderCounters in the same way as the demo app.

In case 2 and 3, there's not really a way to measure this from within ExoPlayer, because if the player was able to measure a sync problem (and we're not performance-constrained, i.e. case (1)), it would just fix it.