The earliest sign is usually a musty smell in the house, because air moves upward from the crawl space into the living space. Underneath, look for white or grey surface growth on joists, discoloured subfloor, and a relative humidity reading above 60%.
Signs you can see without going under the house
A musty smell that is worse in rooms over the crawl space, worse in summer, and worse when the HVAC runs. Cupping or lifting hardwood over part of the floor. Allergy-type symptoms that improve when occupants leave the house.
None of these confirm mold on their own, but together they are a strong indication that crawl space humidity is out of control.
What to look for underneath
White, grey or greenish surface growth on joists and subfloor, most often near the perimeter and near any water entry point. Staining that follows the grain. Fallen insulation, which both indicates and causes moisture problems.
The single most useful thing you can take down there is not a torch but a cheap hygrometer. A reading above 60% relative humidity tells you the conditions support growth, whether or not you can see it yet.
Growth versus decay
Surface growth sits on the wood. Decay fungi digest it. Push a screwdriver into a suspect joist: firm resistance means surface growth and a remediation job; if it sinks, the member has lost section and this is now structural work.
That test costs nothing and changes the scope of the job by thousands.
The bottom line
A musty smell plus a humidity reading over 60% is enough to act on. Confirm with a look underneath and a probe of anything that looks affected.
For a number against your own crawl space, use the cost estimator, or call (910) 886-2018 and we will come and measure it.