That's it really. Was in a large-ish meeting sitting at tables in a U shape. Work colleague who I don't know well (from another team & I'm fairly new) was wearing a perfectly nice, rather loose, knee length summer wrap dress that had ridden up and parted when she sat down. She was sitting quite 'legs akimbo' and slouching in her chair and I could see right up between her legs with lots of inner thigh exposed, to the point I could see the colour of her pants.
All the people on my side of the "U shape" were females so I don't feel so bad not saying anything. I'm not sure how I could have anyway, without making it a bit obvious by going over to speak to her on other side of the room in the middle of the meeting. But if there had been men there I might have done. It was 2 hours long with no breaks - and I'm sure the 4 other people on my side could see too.
If it was me I'd be a bit embarrassed but prefer to know so I didn't do it next time. She left soon after the meeting ended so I didn't have a change to decide whether to say anything. Thoughts?