I think it's hard to know, and there's a grey line between choosing to stay child free, and it just happening.
People also often will say "oh I'm so pleased"/"never wanted them"/"much happier without"/"hate children". When the real term is that they desperately wanted and threw everything at it, and it didn't happen. That's their choice not to share with others.
I know a couple of people who are open about the fact they put their career first, assuming that they'd have a baby when they wanted, left it to late 30s/early 40s and it didn't happen. Both of them are very open, but they generally describe it as they chose not to have children. I know in both cases that they did try, due to conversations that I have had at the time, and I know at the time they were devastated. You wouldn't know that unless you were there at the time.
One of dm's best friends was very vocal about her and dh not wanting children. She even had her tubes tied in her early 20s. I don't think they'd do that nowadays, tbf, but she did. She's now approaching her 80s and recently she admitted to dm that they had changed their mind and she had an operation to reverse the tube tying when she was in her 30s. But it didn't happen, something she's only now admitting she was/is upset about.
Now are those cases voluntary, or involuntary? In each case they made choices that made it much less likely they would have a child.
I only know of one couple who said they wanted to be child free. That's because one of them has inherited a very serious genetical illness. They are having the dreadful symptoms of it, and likely will die in the next 10 years, before they hit 50. Any child they have would have a 50/50 chance of inheriting it. Having watched her dad die from it, they made that decision.
Other than that, I know several child free couples who are unlikely to have children now due to age. I have never discussed it with them, and I would never unless they brought it up. So I think it's very difficult to know, nor is it my business.