Currently there is no automatic service that comes with Steemit. There are, however, a lot of bots developed by users who are checking the Steem-Blockchain for plagiarism such as @cheetah or @steemcleaners. Hope you found my answer useful.
There are groups who continuously check for plagiarized content, They have come together in Discord servers and if you find any plagiarized content you can drop a link in those servers in specified channels for those with the power to flag the posts can do so.
It is not done by any automated systems as yet.
Once a name is on the list for this, those account become easier to follow and track the content, In this manner the system could be classed as semi automated by the membership of the community.
Is it really that hard for people to come up with original content to post? In most cases it is not. sharing just an opinion about something can be a good post, Sharing one of your own pictures can become more profitable too. With smart phones now everyone has a phone in their pocket, Taking a picture takes no time at all from your day and can be done while you are waiting for something else, A bus or dinner to cook. Plagiarism is a cancer in the publication of material. Those who do so do for a quick profit and offer nothing constructive to a community. They rip from a community stealing from individuals and collectives of people. It is the very same as coming in and stealing your drawings from your home, or your book of poetry and getting paid for what you wrote.
Steemit is a user interface and it doesn't check that. But there is a bot called @cheetah that seems to use Google to check if a post includes text from public source. I've seen wikipedia being quoted in Steem posts and have seen @cheetah post a comment mentioning the source. The tone used by @cheetah is not accusatory. It merely states that similar content has been found on the Internet.
Steemit no, but cheetah and steemcleaners yes. These are the two bots that detect plagiarism and flag it.