Based on your other question you posted, I'm assuming we are talking about a Draft and the sequence models were inserted as drawing views and then using property text "indexed reference".
There is no way I know of to get an actual "R1, R2, etc..." list, however if you use the Manage Links dialog, I believe it list the models in the order that they were attached and it updates correctly as views are deleted and added. So just count down the list until you find the model you want and you should have the correct index number.
I suppose that the order could be seen in the Manage Links form in Solid Edge
Just did a test and the entries are in exactly that orer I put them into the draft