Fortunately for us, this stage still exists in FS4SP, and it might help you when testing and debugging your crawling.
In order to enable the spy stage, first stop the FAST configserver
nctrl stop configserver
Second, open up %FASTSEARCH%\etc\pipelineconfig.xml
Typically you want to add your spy stage before or after the custom extensibility. In the example below I have added it before.
After the edit, save your file, and start the configserver up again.
nctrl start configserver
If you watch the %FASTSEARCH\var\log folder during indexing you will see a file named spy.txt appear which contains all current fields available to you.
The file is overwritten by each processed file and will contain information from the latest document only. If you index using only one document processor it’s still a valuable tool during development to check that you are receiving the data you expect for your custom stage.