I can't speak for others on this, but I have always been very open about who I have 'done things with' and still see from time to time as friends with my subsequent girlfriends.

Communication and transparency. I am married and my wife knows, has met and is friendly with all of my ex fwb that i am still in touch with. If i became single and my new girlfriend couldn't handle that i am friends with someone I have previously had sex with, then they are not the right person for me. I am not going to walk away from a totally secure and loving friendship with people I have known for over 30 years in some cases just because someone I met many years later it feels all insecure.

As someone who has slept with a number of people who I am still friends with over 20 years since we last did anything sexual you are 100% right.

Once you have been that close to someone, unless it has dissolved into a dislike and you don't see each other again, there will always be little clues that you will give out in your body language and interactions.

I'm not talking about draping over each other but simple things like just brushing past through a gap without thought rather than asking someone to move a little or if they mind.