This is a question purely out of curiosity.
I have several various PCBs, and I'm curious as to how many layers they use, which helps me decide how many layers to use in my project (for example a cheap competing product using 4 layers suggests to me it is possible to do my product with a 4 layer design).
From looking at the vias (and where they connect to), I can usually determine if they have 2 layers or more, and I can tell apart single layer boards too because they don't have a top or a bottom side, however, I've never been able to actually count the number of layers a more complex board like a motherboard or graphics card uses.
Any clues?