Inky caps are often easy to recognize once they mature, since, perhaps because their gills may be too close together to allow the spores to be ejected, the black spored gills will often deliquesce and turn to ink from the outside edge inward, until only a stem might remain. Species that don't turn to ink have strongly pleated caps from being very thin-fleshed. You can actually fill a fountain pen with the "ink" and write with it. Many of these species are edible, and some people actually eat Coprinus comatus ink. This process can take place anywhere from a few hours to a few days into the life of the mushroom, which means that they need to be eaten quickly after being found. However, 30 seconds or so in a microwave is said to kill the enzyme that causes them to turn to ink allowing you to store them for longer. This may work for Coprinus comatus, but the others are too small and will just turn into a soggy mess.
Inky caps are recognized by dry caps, gills free or almost so and crowded together, black spores, and sometimes an umbrella shaped cap that is taller than it is wide. A universal veil often leaves granular or floccular material on the cap, which is typically not hygrophanous. They are all saprophytic. Most have caps that are strongly striate. Coprinus (related to Agaricus) is the only genus with a ring, but in those related to Psathyrella, the cap cuticles are cellular, meaning that they are very fragile and can break easily in any direction.
1. Coprinus comatus - "Shaggy Mane", recognized by having a large cap much taller than it is wide and being shaggy, not just scaly (and if the cap is missing, by a string running through the hollow stem). The only species with a partial veil (leaving a movable ring on the stem). Remarkably, this mushroom is related to Agaricus. The rest actually evolved independently and are related to Psathyrella.
2. Coprinopsis - usually white to greyish caps (instead of brown), often with shaggy or scaly velar material on the cap (or if not, will be especially large (>5cm) or especially minuscule (<1cm), and usually striate. Found on the ground, wood, or sometimes dung.
One species, C. atramentaria, contains Coprine, the active ingredient in Antabuse. If you eat this mushroom either a few days before or after consuming alcohol, you will get sick, as Coprine will interfere with the body's ability to process the toxins in the alcohol. Another group, the C. lagopus grp, is so short lived you have to be in the right place at the right time to see it during the only few hours a year it will fruit. Fortunately it is incredibly common, and you will find it at least in its inky state all the time.
C. lagopus grp - very short lived, always copious velar material on the cap. Grey. Ground or burnt ground. <5cm.
C. friesii - purple grey, in grass, does not deliquesce as much as C. lagopus. Velar material on the cap. Small, cap <1.5cm tall.
3. Coprinellus/Parasola - usually brownish (at least when young) instead of grey. Strongly striate. Caps usually between 1-5cm and may have granular particles, but only rarely (in the PNW) shaggy or scaly velar material like Coprinopsis. Some species do not liquify. Found on the ground, wood, or sometimes dung (see Coprinopsis for white or tiny dung species).
C. micaceus - glistening mica-like particles on a yellow-brown cap <4cm high (larger than the others). The particles wash off easily. Browner in age than Parasola and C. impatiens.
C. domesticus - larger particles that resist wearing off, orange felty mat sometimes in the substrate.
C. disseminatus - grows in large clusters, small (~1cm). Hairy particles on cap, but does not turn to ink.
Parasola plicatilis/lactea - separated by their bald caps with the finest pleats almost to the centre and not turning to ink. Free gills with a collar. In grass (P. lactea also found in disturbed soil), unlike the C. impatiens group. Brown turning grey except in the centre. Cap <2cm high.