Being able to recognise normal behaviour and appearance of the species that you work with will allow you to confirm that the animals you are killing are likely to be normal, healthy individuals. If you are killing animals during a study, when they may have begun to show evidence of adverse effects, you need to be able to recognise these, so that you can help your research group apply appropriate humane endpoints. Once you are familiar with the normal behaviour and appearance of animals, it is relatively easy to appreciate abnormalities. For example, in the video below, the mice on the left are normal, but those on the right have slightly ruffled or “stary” fur, and early sign of poor welfare.
Much more detail on how to recognise pain and distress is included in Recognition and Prevention of Pain, Suffering and Distress eModule.