The leaf shape is often the easiest way to tell, hemlock is more "ferny" than parsley dropwort. And yarrow is another similar looking weed. After a while you can tell them apart from a distance, we have all three around here.
This website is good:
Wild carrot not only looks like carrot but smells like it too and has a scrawny cream coloured taproot. Parsley dropwort, has multi branching roots, wider leaves and little nodules on the roots every bit which will grow if left in the soil. Hemlock has a purplish tint to the stalk which is hollow and it is tall and smells like mouse pee. Most of the wild yarrow grows very low to the ground and has a fine leaf.
Did you know, that what you thought I said, was not what I meant