Hold it up to the light. If all that's indicated is the surface traces, it's a 1 or 2-layer. You can usually discern internal planes, and most multilayers pretty much block all the light, because they have solid internal copper planes.
The only good way to count layers is to section the board, preferably through some vias.
John
What are the typical methods for determining how many layers a PCB board is. Like after you take something apart and see the PCB ... how can you tell if it's multi layer and how many layers it is. I tried looking at the side profile of the board ... it's FR4 but that's all I can tell by just looking at it.
In the old days yes but not today's PCBs they make them dark with 4 or more layers and may be the way to find out how many layers is to read the schematic or diagram.
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The reason the board is 'dark' with internal layers is you can't see through the internal copper. It's common for at least one of the internal layers to be a ground plane and at least one other a power plane, so virtually all the layer is copper, rendering it impossible to view through.
As John noted, the only real way to know is to section the board at a via (or look at the original design fabrication notes which generally has a stackup diagram).
Cheers
PeteS
Many times, the PCB designers will put a "layer key" on the edge of the board somewhere. The "key" is simply a layer number, made in copper, arranged so that when you look "thru" the board (illuminated from the rear), the numbers will appear adjacent to each other. FR-4 is moderately translucent, so this generally works for up to many layers (depending upon the thickness and materials used). Maybe you'll be lucky and the board will have such a "key" designed in.
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