I'm a harmless cheekbone
How much easier is it to recognize other duals once you have had a dual relationship?
I can spot ESTj fairly well I guess, they usually have a more masculine appearance, they laugh at their own stupid jokes, and they oftentimes wait for confirmation when they make statements as if they are nodding whenever they say something relevant. Also their Te is pretty evident, maybe ESTjs are among the easiest types to spot.
The general question is, once you are 100% sure you have found a dual, how easily can you spot other duals?
Much much easier. I haven't known any Si dominant type growing up but many Se ego types so I was kind of blind to Si. But once I become close friends with an SLI, I realized the goodness of it and so became much better at recognizing them.
I usually think a person is SLI when I notice that he/she is better than many people (in something particular), or have interesting observations but just don't speak or try to stand out, and at first they have that unexpressive look when I'm talking lol but after some time they start talking with me, asking questions, joking...etc. And they usually wear very simple clothes.