I use a 20oz soda bottle that gets sanitized and filled midway through the bottling process and then is stored just like the glass bottles. By squeezing the soda bottle, the rate of carbonation has be roughly tracked. Once it becomes rock hard you're probably just under 3 Vols of pressure and it's time to pasteurize. I like to give it a squeeze a couple times a day mostly because it's fun
even though most batches of mine carb in just over a week.
As always, the pasteurization sticky is a great resource.
As a general rule of thumb I add more yeast if the batch has sat more than 3-4 months in secondary regardless of clarity.