I doubt we all know all of them. But they tend to persist for long periods, and so are easily discoverable by reverse engineering speeches. It helps that some people are quite explicit about it. Either in historical retrospect, as with the Atwater quote here: https://en.wikipedia.org/wiki/Southern_strategy#Evolution_.2...

Or with "saying the quiet parts loud", where people who aren't as good at dog whistling drift into being more explicit. Or people who are explicit get pushed into saying things where the racial connection is more easily denied. The Trump administration's Muslim ban is good example of that. They started out advocating for a ban with strong religious and racial components. When the public and the courts said no, they iterated to something less blatant. They will keep rinsing and repeating until something gets through.

I would guess that the answer is that not all anti-racists know all the code-words. Thug, urban, inner-city and so on are all known because they're the most used.

To contrast, I used to have a couple of friends who would use the term "friend" as a stand in for n* when talking about people in public. I figured it out from being around them and hearing how they stressed it differently than normal, but any random stranger wouldn't have picked up on it.

The same way we decode any phrase that has a coded meaning. I assume you can understand irony, jokes, and satire, dog whistle phrases are no different.