I suppose all carrot family plants look much the same when in flower; just compare this species to, say, hogweed. At first sight there may seem to be little difference but, as always, there are clues if you look closely to enable an identification to be made
The obvious thing about hemlock water-dropwort (Oenanthe crocata) is that you always find it with its roots in wet ground; in ditches, drainage channels, marshes and so on. Whilst there are other umbellifer flowers that grow in these damp conditions they are far less common. Blooming from early May through to the end of July hemlock water-dropwort can be found almost anywhere the ground is usually wet, it does not grow in water as such, it loves mud.
Apart from this habitat preference hemlock water-dropwort always grows in large masses whereas many other carrot species are less communal. It is also quite a large, sturdy plant which makes it different to many of its cousins. Finally, the leaves are very celery-like. The habitat, the tendency to community and the shape of the leaves distinguish it quite clearly from hogweed and its size sets it apart from other members of the family that may be visible at the same time.
This a very popular pollen source for insects, mainly beetle and flies, and each flower head is a mini habitat of its own. Like its cousin, hemlock, it is poisonous too!