How do users who don't have profile on Careers Site see the top percentage of tags they have been active in? E.g. Jon Skeet is top 10% for C#. How do other people see this information for themselves if they don't have a careers profile? Is this even possible without having a careers profile?
There is a technical difference between the rankings on Careers and those on SO top tags. Currently Careers only ranks you against other people on Careers. That being said, the distribution for most tags is reasonably close on Careers to what your top % would be for a tag on the whole of SO.
It is actually on our list to change our ranking to be that of SO, so I would suggest ignoring this difference for now. For those curious why we made this distinction initially, it's because around a year ago, only the most engaged SO users were on Careers, and so our rankings showed everyone in the top 10%, 20% or 30% for just about every tag they had answers for. Today we have many more profiles and we've been tracking the progress of these metrics with the intent to use the SO rankings once it makes sense.
That information is only available on careers profiles.
However, you can see the top 20 answerers in any tag. For C# for example, the following URL shows the current top 20, the top answerers for just the last 30 days, and the same information for questions:
Here you see Jon is not only listed in that ranking, he holds the top slot.
Any tag page has this linked to the top users page, here is the C# page link:
If you yourself have answered questions with this tag, your score will be listed, but it may be placed at the bottom if you don't otherwise are ranked. That way you can at least compare your own score with the top users and see if you are close yet.
For the c# tag, which I don't follow myself, I managed to gain 1 point (probably a Python answer that was also tagged with C#), so I am listed at the bottom. The answer is older than 30 days, so I am not listed in the 30 day ranks at all:
This is of course not the same thing by far, but is also more accurate than the percentage reading given on Careers. On Careers I rank in the 10% for the django tag for example, but I don't rank on that tag's top 20 users page; in fact I have quite a ways to go before I'd ever get there. But for all people listed on Careers that have answers in that tag, I apparently rank in the top 10%. Golly. :-)
You can use Data Explorer to query for this information, like I mention in this answer. Now that the data is updated fairly regularly (weekly), this should give you a reasonable accurate idea of where you stand in a given tag.
Note however that the percentile on Careers is calculated differently, as it's based on your position among users who also have Careers accounts. As such, the Data Explorer figures may be more relevant anyway as they look at the entire user population.