The ability to experience pleasure and pain is the mark of sentience. The available evidence supports the claim that animals are sentient, just as it supports the claims that other human beings are sentient. In other words, we know that animals experience pleasure and pain to the same extent as we know that other human beings experience these sensations.
The clearest example is that many animals demonstrate typical pain behaviour, such as moaning, yelping, writhing, seeking to avoid the source of pain, etc. Just as we may infer that a fellow human being is in pain if they exhibit this kind of behaviour, we may infer that an animal is in pain if they exhibit such behaviour. Furthermore, there is physiological evidence in support of this claim: many animals have nervous systems very similar to our own, and their bodies react much like our own would when harm is inflicted. Overall, then, there is no good reason to deny that animals can feel pain.