I'd say most of the more complex organisms have a particular level of sentience that should be acknowledged. While we might have taken the ability to create tools to an entirely different level than anything this planet has ever seen, there are many animals that have extremely complex social hierarchy, which I would argue requires some sense of "self" vs "others." So while many creatures might lack the ability to express their sentience, I don't believe that is a strong argument for them not having any.
Sentience is a very low bar for a mind to hurdle. All you have to do is be able to feel. Since we can't directly observe feeling even in other people all we have to go on is what our brains are doing. Many animals learn and wire their neural system in a very similar way to humans and this is what causes our feelings (although we can exclude very simple animals like single cell animals). You can feel the neurons in your brain lose their connections when you are sad and realizing there are things you can no longer do and form new connections when you are happily excited as you consider new possibilities. Many animals have complex neural systems that also likely produce an affective experience.
Without a doubt. All one has to do is see the pictures of a military dog who will not leave the grave side of his fallen master, the pet that sits at the kitchen window waiting for their owner to return home, and upon seeing the person, goes into a maniacal fit of elation. Pets can sense when their owners are depressed or otherwise out of sorts, and this sentient characteristic is documented too many times, to allow for much debate.
Yes, I believe that animals are sentient. I was at a zoo a few weeks ago and spent a lot of time at the gorilla exhibit. The baby gorilla knew I was there and she knew I was watching her. I could tell from her expressions when she was having fun, when she was feeling depressed, etc.
There have been much evidence that many animals, particularly lifeforms with higher developed nervous systems are sentient. There was a very interesting case wherein a gibbon monkey in captivity warned humans that her children were in danger and pleaded them to save them. There have also been evidence of many canines and felines showing sentient interactions towards other animals as well as humans.
A dog cowers when you yell at them, they bark and jump on you when you are happy. They are loving and loyal to those whom treat them right. Of course they are sentient. They may not be human, but they have feelings. They should never be beaten or abused!