Recently I discovered that a friend of mine, who I knew for more than a year, is actually interested in FIRE. Can you imagine that never ever before we both touched this topic? It was just a pure accident and only because he actually was pretty open about it online.
This made me think that perhaps there are many more people interested in Financial Independence around me. It is just that we don't get a chance to discover this. I myself never talk about FIRE with "outsiders" and don't even discuss investing much (to be honest there are few people wanting to talk about it anyway). So I am not surprised that others do exactly the same and as a result we never find FIRE friends (even though some of our friends may already be into FIRE).
So how could this discovery problem be solved?
"what?". You might have to explain to them what FIRE means, which might be non-trivial or they will just plainly assume that you are "weird" and that's it. Moreover, in some cases openly signaling about FIRE may be not even an option - e.g. telling your employer that you plan to leave soonish does not sound like a good idea.
Mention 4% rule, but not in an early retirement context. I had some success with mentioning 4% rule as a rule of thumb for general retirement planning like "I've read that you need 25x of your yearly pension saved for it to last for 30 years". Obviously, you shouldn't be doing this out of the blue, perhaps you were discussing investing or retirement planning already. If a person does not know about FIRE, they probably won't attribute 4% rule to it, but if they do, they will suspect that you are into FIRE as well.
Look for proxies. What are the characteristic features of a FIRE person? They probably invest and know quite a lot about it when compared to an average person. They are probably frugal, but not always. They may track stuff and just like numbers and data. They probably like to save money, learn about better ways of doing it and cut spendings. You get the idea. What also good about all of these is that some non-FIRE people do them too. There are just people interested into investments or minimalistic or spreadsheet lovers. So when you talk about such topics, this won't be attributed to FIRE, unless they know the concept. So you can use these topics as a "markers". Do you have a frugal friend who invests? They may be into FIRE.
Use a secret phrase. This one is a joking attempt at doing "stone soup" to create such a phrase. It would be cool to have some phrase, which FIRE people would treat as "hey, I am in the club, we can safely talk about FIRE" and non-FIRE people would ignore. Unfortunately I haven't came up with one. But perhaps you have some ideas? Basing it around 4% rule or Mr. Money Mustache seems like a good idea, but making it not weird to ordinary people (and e.g. avoid them looking it up online) sounds tricky. "What's your asset (stash) allocation?" might be a good start.
Let's say you found someone who has characteristics of a FIRE person, but you haven't done the "come out" yet. I think the safest option is to just continue getting to know them until you build enough of a friendship to just directly ask "have you heard about this thing called FIRE?".
Before that you might be wrong and I don't know whether you should introduce people to FIRE in this way. It is a challenging path and for someone it can be the best blessing, while for others more like a curse.
Alternatively you can just join FIRE communities, which completely solve this signaling problem. This can be an online group, a meetup or a community like firedating.me. Believe me, the site looks confusing enough to non-FIRE people (I had a confused visitor asking me "Wait, so what does 'mustachian' mean?") to make sure that only people in the theme join. Despite the name, friendship search is also supported!