HOW DO YOU KNOW WHAT SONGS ARE LOSSLESS?

If you are listening to music on a device that supports lossless playback and have the option enabled under Settings > Music > Audio Quality, then it will display a "Lossless" banner under the album artwork while listening on the device.

Note that iTunes does not support Lossless playback at this time.

afaik all music is Lossless when enabled (Apple has long-required all music uploaded to iTunes/Music to be lossless). You should see a Lossless logo in the "Now Playing" section too (below the song name in iOS or beside the song-name in macOS) which you can click on to see what the lossless quality is.