I doubt that many people would choose to freebirth and just trust that they are some amazing intuitive special woman who will just 'know' that something is wrong by some instinct that the rest of us don't have.
Most women who freebirth have spent hours and hours and hours reading, discussing, researching, studying the normal stages of birth so they would know when its not normal by being educated about it.
They would also ensure that they knew what steps to take to ensure the safe arrival of their baby if anything out of the ordinary did occur.
I think most freebirthing women know more about birth than average. That might not have been true in the old days, when alot of births were unattended but not through choice, just through circumstance, but women these days have access to so much information they go into it as fully informed as you can possibly imagine.
Thats the impression I get anyway.
And my next birthing will be at home with a midwife sitting in another room having cups of tea and reading magazines. I will sing out if I need her. Its totally possible.
ETA: I do think women in labour have incredible instincts that they aren't usually encouraged or allowed to listen to, and freebirthing would make them more prominent.