Test case from Igor Nekrstyanov:
1. set
JPI_PLUGIN2_DEBUG=1
JPI_PLUGIN2_VERBOSE=1
before you launch the browser,
2. launch browser from the console where env variable are set
3. open browser console window
4. run applet
you should see some output in the browser console window

if you do not want see heartbeat messages set third enviroment variable JPI_PLUGIN2_NOHEARTBEAT=1

Verified on Linux and windows.