There wasn’t a square inch of wood in this attic that didn’t have mold over it!
To identify quickly if your attic ventilation is poor, just pop your head up into the attic hatch and look around. This time of year, moisture freezes to the underside of the roof deck and to the nails poking through where the shingles were installed. That frozen moisture is frost and it’s easy to see just by shining a flashlight around.
On warmer days, the frost that has built up on your roof deck will melt and take the path of least resistance via gravity. The frost that melts on the nails will cause drip stains in your insulation. Large areas of roof deck that melt will cause water to run down the decking and drip off the end of the 4×8 sheet OR run down a rafter and drip at the bottom. These collection points are normally where your ceiling stains start to show up.
If you find frost, then you have as long as the weather stays cold to correct the air flow before the ice melts and causes stains and possibly mold growth. If the weather warms, up, then it’s a race to dry out your attic and minimize the damage.
Obviously, the photo above is of an attic that had very serious attic ventilation issues for years! Hopefully, your attic doesn’t look like this;-)