An obvious way at Ancestry is to ask your match, you may or may not get an answer, but worth trying. At other companies you can see for yourself, but it is hard work trying to persuade people to upload elsewhere, in my experience.
I have access to a few people's results, so I can cross reference like that as well. It is very interesting to see how much DNA is shared between me, my first cousin and others, sometimes the difference is very marked.
Regards Margaret