The photo below is a good one for seeing what a large infestation looks like.
Lots of yellow, even overwhelming the Christmas trees for the moment.
And on both sides of the photo, individual plants stand out, particularly the flower heads.
The stem of wild parsnip is hollow and typically ridged on the larger plants. Those are vertical, lengthwise ridges, but smaller plants often remain smooth-stemmed.
Once the plants are dead, the entire structure dries to shades of brown, and the seeds stand on their stalks like little watermelon seeds.
Another photo highlights a single plant: