Distinguishing manufacturer's PCB wash from user spillage?

I'm repairing a 16 channel Mackie 1604 mixing desk.
On a large area of one pcb there is a distinctive scum pattern where a
liquid has dried leaving a dried bubble lattice pattern. I somehow doubt it
could be due to in-service drinks spill, due to where this patterning is,
but is there a way of distinguishing deliberate at-manufacture from in-use ?

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/



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