I'm not aware of an easy or casual method to determine for sure if a random log found in a wood is spalted and, if so, how much spalting has occurred. The two or more white rots required to cause spalting infect felled damp logs and boards as well as dead sections of living trees, shrubs and bushes. In effect these different fungi set up defined territories within the log or board. The black lines mark the edge of the territory each fungus has staked out for itself. Some sawmills deliberately and occasionally leave felled logs out in the wet to encourage spalting. Choosing the right moment to convert the logs into boards requires skill and knowledge: presumably they look at the log's end for tell-tale evidence of the condition, e.g., random black/blue/grey lines indicative of fungal activity, which to be able to see clearly might involve cleaning up the ends of the logs through sanding or sawing off a thin sliver. Board up a log too early and the spalting won't be advanced enough, and too late means the wood is too soft and rotten to be of any real use.
Unrelated to your question are techniques people undertake to deliberately spalt uninfected wood usually involving various blends of fresh and partially decayed organic matter and things like fertiliser and even extra (somewhat unexpected) ingredients such as sugar and urine all mixed together. This soup, plus some wood, usually ends up in a tightly sealed plastic bag along with warmth, and perhaps some extra water to make sure there’s plenty of moisture for whatever fungi decide to grow.
If you have a particular interest in the subject try an online search for Dr Sarah Robinson, often known simply as Dr Spalting. She has quite a lot of information out there on the topic. Slainte.
