Stu, it's not that easy in RSL5K. There are several ways the inputs can be used.
They can be used directly
They can be aliased to a tag
They can be buffered by moving a whole input word into a DINT, then the DINT used throughout the program
The buffer word can be used directly
The buffer word can be aliased
They can be buffered by having an input going to a bool, then the bool is used throughout the program.
Finding out if they are used depends on how they are used. If they are used directly, you can cross-reference the input word (Local:2:I.Data) and it will return all the instances of that word. So you could then see if .0, .1, .2 and so on, are used.
In our machines, we take an input word and move it into a DINT. So Local:2:i.Data would be moved into InputSlot2 (DINT). Then we alias individual bits of InputSlot2 to tags. For example a proximity sensor called PX1 that is wired to input 0 on slot 2: Tag = PX1 - Alias = InputSlot2.0.
In our system, the easiest way to find unused tags is to cross-reference InputSlot2 (which will return a lit of results), then I can see what bits are unused in that word. I then create a new tag and alias it to the unused bit.