Techniques to find out which species are endangered
Based on the IUCN Criteria for endangered status there are two broad categories of models to help in determining endangered status. Note that no one methodology is used to determine the status of any species. Rather, a panel of experts consider all available evidence (including numerous modelling studies) and essentially make a determination based on their own judgement.

Range Estimates
Estimating the range of a species uses two primary techniques. The first is an expert range maps drawn by people very familiar with the species distribution, which involves essentially no modelling. The 2nd is ecological niche modelling, aka. species distribution modelling. See 1 and 2 for introductions on the latter. Note that the IUCN guidelines have criteria based on total range size, and well as the amount of fragmentation and patchiness of the range.

Population Size Estimates
Since it is usually impossible to count all individuals of a species, models are used to estimate total populations. These usually fall in the realm of mark-recapture models. There are numerous variations on the basic model form which account for things like an open population, varying birth and death rates, and varying capture probabilities.


1: Elith, Jane, and John R. Leathwick. &quot;Species distribution models: ecological explanation and prediction across space and time.&quot; Annual review of ecology, evolution, and systematics 40 (2009): 677-697. https://doi.org/10.1146/annurev.ecolsys.110308.120159
2: Araújo, Miguel B., et al. &quot;Standards for distribution models in biodiversity assessments.&quot; Science Advances 5.1 (2019): https://doi.org/10.1126/sciadv.aat4858.