You could use TriggL or TriggJ to set a value like a group output (can be sim group) for each motion. Or make your own MyMove routine into which you pass a value to set a num value.
May I ask what the purpose of this is ?
While you could create your own "MyMove" instructions, or you can take it another level and create "ghost" instructions that is an exact duplicate of the original but with your own code inserted to allow you to do this tracking... the downside is that you're now also responsible to do all the error handling which is usually a royal PITA and with your own instructions (non standard) you're also dealing with the programming aspects of things.
So, while you're solving your initial problem, you have created multiple others.
If this is meant to be in an production environment I for one would find other ways.
Additionally, with this approach the read-ahead might (most likely will) trick your "tracker" since it'll show the last robtarget "read" not necessarily which one is the current one being addressed by the motion planner.
