I can usually tell fairly quickly, especially in one-on-one interactions. A couple of weeks ago we had repairs at our house, and the technician was SLI, and I was pretty sure within less than a minute of meeting him that he was. SLI women are harder for me to spot than SLI men.

It took me several years of studying socionics to be able to do this, and trust myself without a bunch of second-guessing, but now I can. If not for knowing socionics I would probably think it was "chemistry" or something.