A DEVONthink 'item link', which uses the x-devonthink-item:// pseudo-protocol, can be used to obtain the source item's ID. The item link looks like this:
x-devonthink-item://5349A993-377B-4F63-9E43-74317939529B
The long code string is a DEVONThink unique ID for that asset. Thus the actual item ID is:
5349A993-377B-4F63-9E43-74317939529B
This is the ID needed to use in a DEVONThink watched group.