Are you? If one can surprise oneself or learn about oneself then one's mind is greater than its apparent working knowledge of itself. Are you fully aware of your own mind? IMHO to the nascent human mind the answer to your question is "probably", their body is the same shape as one's own body, they react to things the same way, hot and sharp are painful to them too, food is good etc, they can tell one things with their actions or by gesture or language that make sense to one's consciousness, so it is likely they have a similar consciousness to oneself, but that does not mean identical. Some seem more similar to oneself than others. In educational theory this process of becoming aware of others is regarded as a developmental process and is studied as the "theory of mind".
One apparently has no direct awareness of others' minds since they can act in ways one did not predict, unless of course humans evolved protean behaviour, like flies, as a means for obfuscating prediction and evading telepathic predation. So while it seems likely that one can only deduce the existence of another mind and its character by inference from sense data, we cannot be certain we are fully conscious of all types of sense data being employed. Though whatever the kind of sense data involved one can still only guess at what others think, as a reflection within one's own mind and as a projection of one's own thoughts.
So while consciousness in others can only be attributed as an extension of one's own by sympathy this does not mean this consciousness does not really exist in the same way that imaginary phenomena like fantasies do not really exist, because the nature of other's minds is infered from sensation not invented (unlike Tinkerbell). So it seems likely the capability to sympathise and project have evolved to permit prediction of others' behaviour, because this is the prediction of something real, something sensed in our environment which acts autonomously and which can directly influence survival and reproduction, be they spouse or mother in law!
But if one's own selfconsciousness is incomplete, how then can one's awareness of others ever be complete? Conversely one can sometimes see in others truths about human nature which one denies in oneself yet which must be in fact a part of one's nature for one to be able to conceive of it.
This is why I studied animal behaviour as a zoology undergrad, in part to learn about human nature, because everything one sees in animals, as in others, is subjectively a reflection of oneself and objective scientific data about animals and conclusions abstracted from it are instructive in the principles of evolution which in all probability shaped my human consciousness. Thus by sympathy on the one hand and principle on the other, all the natural inhabitants of the planet can be a muse, in a way that another human being may not, since with animals one may remark on truths that social and political constraints prohibit revealing in the contemplation of human nature.
That is where I ended my answer. To sum up my perspective on this, one's awareness of other peoples' consciousness is directly related to one's awareness of one's own consciousness and vice versa, one's awareness of one's own consciousness is directly related to one's awareness of other peoples' consciousness and this is because we have evolved to use our own brain to as a model by which we may understand other peoples' brains, which is the basis of sympathy, the inevitable and inescapable result of millions of years of social evolution.
What we choose to do with our faculty for sympathy is a moral / socio-political decision each of us must make which can have a profound influence on the course of our lives and this too is inevitably subject to the process of natural selection. I wonder where it will take the human race a million years from now?