Telling a moss from a lichen or liverwort is one thing; naming the actual species is another, and it is where a lot of people stall. You do not need a laboratory, but you do need a hand lens and a few habits.
First, confirm it is a moss
Before naming a species, make sure you are looking at a moss at all. Lichens are grey, crusty or branched partnerships of fungus and alga; liverworts are often flat, lobed and ribbon-like, or leafy in strict flattened ranks; algae is a formless green film. True mosses carry small leaves, usually only one cell thick, arranged around a distinct stem, and many bear the little stalked capsule that gives the group away. There is a fuller account in telling moss apart. Once you are confident it is a moss, the work of naming it begins.
Get a hand lens
A simple folding lens at ten or twenty times magnification is far and away the most useful tool, and it changes everything. Mosses are small, and the features that separate species, the shape of a leaf, whether it has a midrib, the form of the little capsule, are simply invisible to the naked eye. Hold the lens close to your eye and bring the moss up to it, in good light. A drop of water on a dried shoot will spring the leaves open and make them far easier to read.
The features that matter
- Growth form: does it grow in upright tufts and cushions (acrocarps) or in trailing, branching mats (pleurocarps)? This first split narrows things enormously, and it is explained in acrocarpous and pleurocarpous mosses.
- Leaf shape and arrangement: long and narrow, oval, pointed, blunt, swept to one side, spirally set or in ranks.
- The midrib (nerve): a single vein running up the leaf, present or absent, single or double, reaching the tip or stopping short.
- The capsule: upright or nodding, round or cylindrical, on a long or short stalk, and with what kind of lid. When present, capsules are full of clues.
Let the surface guide you
Where a moss is growing narrows the field before you have even reached for the lens. Tight grey-green cushions on the top of a wall or in mortar are usually the drought-hardy screw-mosses and pincushions. Flat glossy mats on bark and fence posts are very often plait moss and its feathery relatives. Deep soft cushions on acid woodland soil suggest bun moss or broom fork-moss, while fern-like fronds spreading over a shaded bank are likely tamarisk moss. The silvery moss in trampled town pavements is a species in its own right. Knowing the usual residents of each surface puts you half way to a name.
Use a key and record it
With those observations you can work through a moss field guide or key, which leads you by yes-or-no questions to a name. Photograph your finds, note where and on what they grew, and consider logging them with a recording scheme or a naturalist app; verified records are genuinely useful to science, and the feedback sharpens your eye. Start with the common species on the species page and build from there.
Go slowly and expect to be wrong
Nobody names every moss on sight, and even seasoned recorders reach for the lens and the key. A handful of common species look much alike until you check the midrib or the capsule stalk, and a few can only be settled under a microscope. That is no reason to be put off. Learn a dozen common mosses well, get into the habit of noting the surface, the leaf and the capsule, and the harder ones slowly fall into place. Naming a moss simply as a plait moss or a haircap is plenty to start gardening with, and the finer distinctions come with familiarity.