I presume you’re talking about two people interacting in real time, there being some general chemistry, how would one bridge the question?
First, there’s usually some indication in the other’s personality or tendencies. People predisposed to entering into FLR don’t often come across as indifferent about their nature. Having any worthwhile insight skills you can pick up on dominance or submission in that nature. That said, a lot of people try to be very politically correct in public with someone they like and don’t really know that well so they may not give any indication, especially if they’re worried it won’t be received well.
Speaking for myself, I’m comfortable with who I am and so don’t have a problem being me. I open and hold doors, defer to her on decisions on what we’d do while together, ask her what she wants if I don’t know, etc. If she asks me to decide, I will because she told me to. Generally I try to be good company, be helpful, do what I can to make her happy and show that it matters to me that she is. I try not to make it about money or sex, those things take care of themselves if the relationship has the foundation it needs, and largely decisions about those wouldn’t be mine to make if the relationship comes into being.
This usually gives the Lady a pretty good idea who I am and what I’m probably about, then if she’s aware and especially if she’s seeking an FLR, it comes up in conversation. Most often they’re not aware or not seeking, but they enjoy the attention, which is great. When it has been brought up, it was usually a Lady who was indeed aware, but already had someone in her life.
If all else fails, I’d have no problem with bringing the subject up. Politely. Privately. At an appropriate time when context allows that it’s not coming out of left field. If I know someone well enough to like her and know her opinions on major issues (the standard deal breakers) to think we might be otherwise compatible there are ways to gently open the subject up to discussion and if nothing else it can be a fun chat. And even if she’s new to the idea, she may like it, or even like it much.
It’s not really all that taboo these days. Aspects of it can be taboo, but you don’t have to start there. Find out how much of this there is in common, how much interest is there in the other person.
Life’s not short, but it’s too short to waste time not searching out the things that matter to you.
Now, for the Lady’s end of this I can’t really speak, but I imagine it’s much the same. Sending out signals to see how he reacts and/or just broaching the subject in some appropriate way.
The harder part has always been finding someone who shares my views on important topics, wants the same things in such a relationship, and then both of us being fully available to actually begin the endeavor. Life is screwy how it finds ways to interfere. So really, nothing ventured is nothing gained because you never know.