How to see the difference between parsley, coriander (cilantro) and celery
For years I have been unable to notice the difference between flat-leaf parsley, coriander (aka cilantro) and celery, unless I tasted or sniffed them. Since they are all green, have about the same shape of leaf and usually come wrapped in plastic bags, I needed to find some unique visible characteristics of each them to distinguish one from the other.
So here is your little almost-identical-looking-herb identification guide. Each herb has different varieties that vary in shape, size and colour, so this guide only uses characteristics that are shared by all varieties of the herbs.
Coriander is probably the easiest to recognise, as its leaf shape is clearly more rounded than that of both parsley and celery. Also, the coriander (aka cilantro) varieties that I know usually have leaves a shade of green lighter than most parsleys and celeries. So if the shape of the leaves is not pointy, you can be sure that it is coriander.
To see the difference between the two pointy-leaved herbs, one should look at the stems instead of the leaves. Whereas parsley always has a smooth stem with a more or less circular section, celery stems have a jagged surface and a crescent-shaped section. Celery stems look like a small version of the celery stalks.
Extra: For those that want to see the difference between chervil (a not-so-widely available but very aromatic kitchen herb) and the herbs above, the following might help.
Chervil distinguishes itself by its smaller leaf size and the higher number of ramifications (branch junctions) on the route from the main stem to a leaf. Whereas parsley and celery plants always make a leaf after one, two or three (very rare) ramifications, almost all chervil plants branch out three to four times before making a leaf.
Please use this identification guide on your market or in your own kitchen only. Do not use these characteristics for the identification of wild herbs as there are many Apiaceae that look just like parsley, but are deadly poisonous. Like this one for example, which supposedly killed Socrates.