Hi Grimente

This species has several varieties, each of which has its own characteristics, including the shape of the leaves and teeth.

You can enter the Ipni website (https://www.ipni.org/) and enter the scientific name of this tree (Alnus glutinosa) in the search field. There you can see the names of its different varieties and look for their descriptions on another site called BHL (https://www.biodiversitylibrary.org/), and easily separate and identify them by the authors' writings.

these are good advices

beware, however, that there is a tremenduous variability of leaf shape and size, on the same twig, between different twigs of the same tree, and of course between individuals. Sprouting shoots after pruining are particularly tricky. Several botanists have described forms on the basis of a few aberrant leaves, but sure this variability also is interesting to investigate.

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Alders are members of the birch family (Betulaceae),

The easiest way of recognizing an alder tree is by its distinctive little fruiting body, called a strobile. They appear in fall and look like 1 inch (2.5 cm.) long cones. ... The female flowers on an alder tree stand upright at the ends of the twigs, while the male catkins are longer and hang down.

The red alder (Alnus rubra) tree is a large deciduous tree with small oval, brown wood cones, ovate leaves, and mottled light gray smooth bark. Its spreading branches create a conical shape with a slender, rounded crown.

The black alder (Alnus glutinosa) is a tall, medium-sized deciduous tree with rounded glossy green leaves and dark-brown hard conifer-like cones. Black alder bark on immature trees is greenish-brown that gradually becomes dark gray and scaly.

You just use descriptive morphlogy terms, like serrated, trifoil, etc. :)