To answer the first question, a SARGable operation is not necessarily one that is working on an indexed column. A SARGable operation is one that would provide a better result if the column were indexed. So you don't need to know if there is an index to know it's a SARGable operation.

To answer the second question, I'd say anyone whose spent a few hours researching performance tuning in SQL server would have a pretty good idea. That's the whole point of SARGable operations -- they've been identified by people who know how the optimizer works as ones which generally enable the query optimizer to use an index to improve performance. You identify them first, and then you know where indexes would help (or not, as the case may be.)