Lichens are extrordinarly diverse in their growth forms, ecology and biochemistry and lichens which look alike are not necsaarily closely related. Therefore, the initial approach to identification involves recognising the growth form or type rather than it's taxonomy. This is followed by a close examination of the main body of the lichen, this is usually the vegetative body or thallus, to look for modified structures such as rhizines or cilia and the reproductive bodies. These are the apothecia and perithecia which produce the ascospores and the structures such as isidia, soredia and pycnidia which enable the lichen to reproduce asexually. Their presence and their size, shape and structure are important characteristics in species identification.
The biochemistry of lichens is equally complex as they have the ability to produce over 600 compounds, most of which are unique to lichens and often associated with specific lichen groups or species. Many of these compounds can be identified by simple chemical tests, often referred to as "spot tests" and can often be an important component in the process of identification
In addition to the morphological and chemical characteristics, the habitat and substrate are also important elements in the identification process. Unlike fungi, lichens do not appear to form a close relationship or extract any organic nutrients from the substrate. However, many lichens are restricted to certain substrate types, and it is important to record the nature of the substrate e.g. type of rock, species of tree.
Not all species can be identified in the field, some will require a microscopic identification of the structure of the anatomy of the thallus, reproductive structures or spores. However, much can be achieved by a careful examination using a hand lens combined with the spot tests.
There are some excellent field guides and basic identification keys available and help is available from various internet forums and websites. However, the best way to learn about fungi is either to join a local fungus group or attend one of the excellent courses organised by the Field Studies Council or the British Lichen Society.