The Input Source ID can be found in Karabiner's Event Viewer (described here).
This information is missing in Karabiner-Elements' Event Viewer. In fact, all Input Source info are missing.

One workaround: Are you trying to get the ids of external USB keyboards/mice? If so, you can get them from the command line with:
ioreg -p IOUSB
I save that as an alias for quick access:
alias usb="ioreg -p IOUSB"
@nguyenhimself Not the ID of a device but ID of an input source as English Language layout or sat French.
I need that too.
You can investigate input source information in EventViewer > Variables tab with Karabiner-Elements latest beta version.
How to upgrade to the latest beta version:
https://pqrs.org/osx/karabiner/document.html#check-for-updates
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Most helpful comment
You can investigate input source information in
EventViewer > Variablestab with Karabiner-Elements latest beta version.How to upgrade to the latest beta version:
https://pqrs.org/osx/karabiner/document.html#check-for-updates