We can identify whether or not something is a parasite by examining the nature of its relationship with its host. A parasite is an organism that lives off of another organism, known as its host. The host provides the parasite with food, shelter, and other resources, while the parasite in turn takes nourishment from the host, often at the host's expense. This relationship is usually considered to be harmful or dangerous to the host, and may even be fatal in some cases.
One way to identify a parasite is to look for signs of damage to the host. If the host appears to be suffering from an illness or injury that is not seen in other animals of the same species, this may be an indication that it is being parasitized. Other signs may include weight loss, changes in behavior, and decreased productivity.
Additionally, we can look for physical evidence of a parasite on or in the host. For example, if a host is infected with a tapeworm, eggs of the parasite may be found in the host's feces. If a host is infected with fleas, they may be visible on the host's skin.
Finally, a laboratory examination of the host can be used to identify the presence of a parasite. This may involve testing a sample of the host's blood or other tissues for the presence of parasite proteins, or looking for parasite eggs or larvae in the host's intestines.
In summary, we can know if something is a parasite by looking for signs of damage to the host, searching for physical evidence of the parasite, or performing laboratory tests on the host.