FAA has really done very little on the issue
That's really not very fair, to be honest.
For example,
Amendment 121 to FAR 25 is a pretty comprehensive rewrite of much of the relevant parts of FAR25, and the result of a long and involved consultation process. (And involving not just FAA but other authorities too; these kind of new rules have to be harmonised...)
The so-called "Appendix X" to account for SLD is still in development, it is true; but while a lot of research has indeed been done, it's nowhere near clear that there's a full understanding of the phenomenom or how to address it practically.
As to the original question
Wonder how the pilot can tell what size the supercooled drizzle drops are?
You can't tell how large they are, but there can be indications to the crew that you're in conditions outside the approved icing envelope. For example, due to their greater size the SLD drops impinge on different parts of the airframe to "normal ice"; observation of an unusual ice pattern may be indicative of "extreme/unapproved" conditions and the cue to get the Hell out of it....