Hey Justin that is a good question man, I would say the best way I go about determining this is somewhat off of feel based off of how demanding / taxing the first working set was. For instance in my opinion if you do a true all out rest pause set you have given everything you have to give there and its best to just move on. If I do a straight set then I know I can give more with an additional straight set. The way that plate loaded press worked out was on the first set I got 10 full reps on my own but I knew in order to get 11 it would have to be a forced rep so rather than that forced rep I chose to shut it off at 10 and get an additional second set. The more you work with true sets to failure the more i think you will be able to gauge this on your own, hopefully this helps!
Suraj – Thank you!