Imagine you are Sherlock Holmes. You have a case to solve. You need to find out if a population of species is exposed to pollutant(s). What can you do?
You could directly look for pollutants in the population
By taking the blood, tissue, or waste of some organisms in the population, you can go to a chemistry lab and ask them to conduct an analyses on the sample. The lab would probably use simple or fancy instruments and methods to look for presence of pollutants in the sample.
You could look for effects of pollutants
A biomarker is defined as any non-typical biological response to a chemical. For example, a biomarker can be a response like inactivation of an enzyme, egg shell thinning in birds, behavioral changes in insects etc. Thus, you can see if a population is exposed to a pollutant by looking for biomarkers.
You could see if the community’s composition has changed
A community has populations of different species. When a community is exposed to pollutants, usually the population of some species increase, the population of most other species decrease, and that of some others remain stable. For example, when excessive fertilizer is introduced into water bodies there is excessive growth of algae. This depletes the oxygen levels in water, killing fish. Pollutants can thus be monitored by tracking the abundance of species in a community over time.
You could measure levels of pollutants elsewhere
For example, in the UK, there was a decline in lead concentrations in the air following reduction in permissible levels of lead in petrol (gasoline). This was matched by a decline in lead concentrations in plants. Thus, by measuring levels of pollutants in other organisms that are interconnected to yours, you can find out if your population was exposed to the pollutant.
You could look for resistance
This is a bit convoluted but bear with me. Resistance means the ability to be unaffected by a pollutant. It develops from continuous exposure to the pollutant. There are strains of plants that are resistant to metals, strains of insects that are resistant to pesticides, and so on. Thus, you could potentially treat individuals in your population with the pollutant you suspect it to be exposed to. Similarly, also treat a population you know isn’t exposed to the pollutant. Then you could compare the responses of the two populations. If your population is less affected, it probably means it has gained resistance to the pollutant — it is or was recently exposed to it.
Principles of Ecotoxicology (4th Edition) by C.H. Walker, R.M. Sibly, S.P. Hopkin and D.B. Peakall