Hemlock in Kit Carson park, Escondido
Ah, the famous plant that was used to kill Socrates. Hemlock plant belongs to apiaceae (or umbelliferae) family. Also known as carrot or parsley family. Umbelliferae means "umbrella" because the majority of species form clusters of small flowers that look like little umbrella. The thing about this family is that lots of species do indeed have family resemblance and it mights be difficult to tell one specie from another. What adds to confusion is the fact that lots of them are edible while others are poisonous. Hemlock is the example of the latter.
Hemlock flowers
The plant that resembles hemlock the most is wild carrot (also known as Queen Ann's lace or Daucus carota). Unfortunately I don't have a picture of the latter for comparison. Actually I have not seen that many hemlocks around but there was one patch where it was flourishing together with other bunch of plants.
Hemlock leaves
Remember that hemlock is highly poisonous and even a little dose might kill you. Be very careful and make sure to identify any plant of the carrot family correctly as lots of them are very good medicinal plants, for instance angelica or spices like coriander.
There are actually two genera commonly known as hemlock: Conium that is discussed in this article and Cicuta (water hemlock). Both are highly poisonous, Cicuta is even more deadly than Conium.