The stains would be at the crotchal region. Men would notice.
I actually did have a guy friend notice. We were sitting cross-legged, facing each other, and I saw him looking at me funny. I looked down and saw a small reddish smear. I looked up at him, and he said, “Is that what I think it is?” It wasn’t (it was paint from a fence I’d climbed) but it looked a lot like a period stain.
What do you do when you’re finally made aware?
Depends on my options. First resort: Obscure it with a long jacket or something tied around my waist. Second option: Wash it out. Last resort: Flee home.
What if you don’t have spare clothes handy?
Sucks to be you. Depending on circumstances, you may have to just bear it.
If someone else noticed would you want them to tell you before the whole world sees?
Yeah, but I couldn’t hold it against anyone who noticed but was to embarrassed to tell me. Besides, in my mind, one person noticing = whole world noticing.
Would it matter if it was a guy or a gal that noticed?
Not really. Guys would be operating from a position of empathy; girls, sympathy.
How should that person approach you?
A discreet, “Excuse me, your pants/skirt are stained” is fine.