@erelas - In many cases it is obvious that the content is not theirs. For instance:

• A post's author has 10 images all taken from different sources, it is safe to assume that some (or all) of the images are stolen.
• The author has their name somewhere on their SteemIt profile, and the poster of the image has a different name.
• The image is widely circulated on major news sites, and the author appears to have no professional affiliation with those sites.

There are very rare cases where the original poster of the image has reposted it from another source to SteemIt and their identity is not obvious, but in general that is not the case. Most photographers will make their identity well-known as they want to gain cross-platform popularity and recognition.

It doesn't hurt to check, but 99 times out of 100, when you suspect content theft, it's actually content theft.