I too agree with using a PET bottle to test for carbonation. One thing though, how fizzy do you want your cider. A very little, little, 3 volumes of carbonation like the "average" home brew, or do you want to have "soda" like carbonation? If you want soda like bubbles, get a new, unopened bottle of soda, and when your sample is as difficult to squeeze as the "factory" bottle, time to pasteurize. As far as knowing how partially carbonated your cider is, you really don't, so your best guess is the best guess. When I pasteurized my cider, the bottle still had plenty of give to it, and the carbonation was much less than I had hoped for. If Yooper sees this, I now have a question about back sweetening. Would you sweeten to taste and then add priming sugar? Would that be an accident waiting to happen? Would you say initially with all the "extra" sugar, will the cider carb faster, needing to be pasteurized sooner? I don't have a clue on that one.