In a general sense: alertness; awareness of oneself and one's surroundings; the capacity to absorb new information, and if necessary modify one's previous framework of thought to accomodate the new information (rather than ignoring or distorting the new information to accomodate one's previous framework of thought). Essentially a 'child-like' state of mind, incorporating such 'adult' insights as are useful.
Or, of course it could simply mean the state of being 'awake'.
I think I and others often use the word in contrast to 'unconsciousness'; when we speak of 'conscious' reggae music we are talking about lyrics that are designed to elevate the mind, inform, provoke thought, encourage the spirit, etc; things which appeal to the highest common denominator rather than the lowest. When describing someone as 'conscious' I might mean that s/he seems more generally knowledgeable about certain things (Black people's IanIstory, the current state of power relationships in the world, how the system works, well-informed on current developments and actively thinking about and/or working on ways to tackle systematic ills) than the average person. So, I think how I use the word sometimes is more as a contrast to ignorance, rather than a really defined concept in itself.
By extension to the description of consciousness in my first paragraph, "alertness", "awareness", "ability to absorb/process information" does not necessarily only apply to "information" that is accessed by the currently recognized five senses. I've had several experiences that indicate to me that there are additional ways to receive information/ perceive things. So consciousness could imply a state where one is able to receive and process the fullest range of information that the human brain/mind is capable of receiving and processing, which may be much more than people generally realize at the moment.
How do I know someone is 'conscious'? By the "contrast with ignorance" definition of consciousness, I would know by interacting with the person, by observing their words and deeds, how relatively conscious the person is. (One thing I just noticed is that when I use the word in some contexts I am automatically putting a value judgement to it; "more conscious"="better"; but really it has to be as much about what you DO with your consciousness, as what you are conscious of; you could be conscious of the way the system is set up, and use that consciousness to re-inforce the wicked system rather than undermining/replacing it).
By an expanded definition of consciousness as in "aware of the full possible range of information available to humans by all avenues, five senses and otherwise, and able to absorb/process said information", I don't know how I would recognize such people. Or if such people exist (who are aware of the FULL POSSIBLE range). Intuition (whatever THAT means) and "vibes" can give me a hint that people have perhaps reached closer to that definition of 'consciousness' than, say, me. Perhaps there are other things that could tell me this but I'm not aware (conscious) of them.
Also I think there could be many different levels of consciousness within the same person: a person could be much more conscious than most people of "beyond five senses" information, but less conscious than most regarding such things as crossing the street safely or counting money. Or, a white person could be more "conscious" (know more information) about Black history than a Black person standing next to him/her, but s/he could never have the level of "consciousness" that the Black womb/man has of being Black, even though that person may know very little Black history.
Well, those are the first thoughts that come to mind thinking about "consciousness."