Monitoring Intelligent Build to Lossless can be done via the Graphics status indicator:

If more in-depth monitoring is needed, an in-session Intelligent Build to Lossless visual indicator can be enabled for the session.

This indicator is turned off by default. When enabled, it will display a visual indicator in the configured corner on each of the monitors that have been switched to build-to-lossless. 

Configuration

Open the Registry Editor using regedit on the Run command.
Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\Graphics\
Create a DWORD key by the name SB2LIndicatorLocation and set the value.

The value will dictate where the Intelligent Build to Lossless indicator will be displayed:

SB2LIndicatorLocation = 1, position of the Indicator is in the top left of the screen
SB2LIndicatorLocation = 2, position of the Indicator is in the top right of the screen
SB2LIndicatorLocation = 3, position of the Indicator is in the bottom left of the screen
SB2LIndicatorLocation = 4, position of the Indicator is in the bottom right of the screen
SB2LIndicatorLocation = 0 to disable the indicator

Apart from the corner where to display the indicator, the position and size are fixed and non-configurable. 

When using multiple monitors, the indicator will appear on all monitors where Intelligent Build to Lossless is active.  It will show which parts of the screen are encoded lossy by indicating red and which parts are already lossless, denoted by a green colour.

Please note that the indicator will only be displayed when Intelligent Build to lossless is active. 

When screen activity stops, Thinwire will switch out of build-to-lossless again and return to Thinwire+ encoding, also known as Selective H.264 (or H.265/AV1 when available). The Intelligent Build to Lossless indicator will also disappear.