She doesn't sound very nice! And she's taking bollocks.
I had my first baby at 30. I had a very strong abdominal core from 10+ years of sit ups religiously every day. Went back to my normal weight (8 stone) and abdominal flatness within days. I had no stretch marks. My abdominal muscles were slightly separated but you couldn't tell this by looking - you would have had to dig your fingers in to check!
Last baby at 40. Again no stretch marks. My "bump" took about 3 weeks to deflate. I never lost the last stone of baby weight (but I'm still only 9 stone) but it's spread out, not concentrated only on the belly, and I still have a strong core which has kept it fairly flat.
I would say that the skin wrinkles in a crepe paper effect in that area, so there is some evidence that it's been stretched, but again I have to poke it with my fingers to see this. Plus I am 46 and my skin is losing elasticity everywhere! This wasn't evident until my late 30s I'd say.
I know lots of women without children who carry belly fat (at any age) and I agree with what a PP said that as women approach menopause and their muscles start to sag, this can manifest in a distended abdomen which is possibly more pronounced if it's previously had a good stretch (and you haven't habitually done core strength work) but by no means happens only to women who've had children.
Sometimes I've observed women in swimwear who have an obvious "apron" from a c-section despite being otherwise slim, or really pronounced stretch marks, but this certainly isn't always the case.
So I don't think your friend can "always tell" at all. She sounds insufferably smug.
If a woman has had a natural delivery you could probably tell by her cervix (mine is a "right mess" according to my smear test nurse!) But that's not something you go around showing all and sundry. You'd have to have your legs akimbo and a speculum in for a start!