Play something that transcodes then check the server log files (Settings → Troubleshooting → Download Logs).
Example: Playing a movie with the video transcoding.
Transcoder temporary directory: c:\temp\plex\
The \plex-transcode-xf…\ directory is deleted when I stop playback.
From Plex Media Server.log:
Jul 27, 2021 22:01:05.511 [9792] DEBUG - [Transcode/xf59brrfeyd2wgscipmfqaj1] Exists("c:\temp\plex\Transcode\Sessions\plex-transcode-xf59brrfeyd2wgscipmfqaj1-03bb0c5f-a547-47e8-a04d-c3646738ec4f\init-stream0.m4s") => 1
Jul 27, 2021 22:01:05.511 [4344] DEBUG - Content-Length of c:\temp\plex\Transcode\Sessions\plex-transcode-xf59brrfeyd2wgscipmfqaj1-03bb0c5f-a547-47e8-a04d-c3646738ec4f\init-stream0.m4s,c:\temp\plex\Transcode\Sessions\plex-transcode-xf59brrfeyd2wgscipmfqaj1-03bb0c5f-a547-47e8-a04d-c3646738ec4f\chunk-stream0-00001.m4s is 6846835 (of total: 6846835).
Jul 27, 2021 22:01:05.511 [9792] DEBUG - Content-Length of c:\temp\plex\Transcode\Sessions\plex-transcode-xf59brrfeyd2wgscipmfqaj1-03bb0c5f-a547-47e8-a04d-c3646738ec4f\init-stream0.m4s is 899 (of total: 899).
Jul 27, 2021 22:01:05.511 [2220] DEBUG - Content-Length of c:\temp\plex\Transcode\Sessions\plex-transcode-xf59brrfeyd2wgscipmfqaj1-03bb0c5f-a547-47e8-a04d-c3646738ec4f\init-stream1.m4s is 738 (of total: 738).