Having had the privilege to meet a number of plant experts during my lifetime, I have learnt that practically all of them use field characters to identify plants, at least down to a family or genus level. Often plant experts are also able to identify plants (at least to a family level, which facilitates later checking) without flowers or fruits. This requires either a lot of experience, a measure of intuition or both. Often, these people will tell you that the specimen in question has the “GISS” (General Impression of Shape and Size) of a particular family or just has ‘the look’. And such an ability to note characters is very hard to explain but can be developed to a workable level of proficiency with the willingness to pay an attention to detail. These field characters may include details of the habitat, habit, bark, smell, leaf venation, presence of hairs on the leaf or twigs, the types of hairs, presence of glands on the leaves or just about any small detail that is available!
Leaf Whispering – when leaves are all you have (on some twigs)
So lets assume we have a plant that is not flowering. The most important thing to look for the leaf type (compound or simple), leaf arrangement (opposite, whorled or alternate) and the presence of stipules. Following that will be the presence of teeth on the leaves/leaflets and a few other leaf and venation details. Just the various combinations of these will allow you to make an educated guess of what family a plant might fall under.
Many laurels like “waxing”! See their waxy leaf undersides. Don’t mistake wax for scales (see below). If you smoke and have a lighter handy (always good to have one handy even if you don’t smoke), apply some heat on the leaf undersurface to see if it melts.
Bronzey or silvery scales (easily seen through a hand lens) on the leaf underside makes the leaves of Croton insularis appear like it is silver coated. This can be distinctive for some plants (eg. Argyrodendron spp., Aglaia spp. etc)
Also look for oil dots in a leaf by looking at a leaf against a light source. A hand lens can be useful as some oil dots can be very small. Many plant families regular have oil dots (many Rutaceae, Myrsinaceae and Myrtaceae, some Lauraceae)
This might seem like advanced leaf whispering but observing how the venation is like is highly useful. Some plants like to loop their veins, sometimes very far inside the leaf, sometimes near the margins. Loopy veins seem quite characteristic of many Annonaceae, Eupomatia, Lauraceae, Monimiaceae. Interestingly a lot of rather ancient plant families.