Basically it’s a big red bar across the top of the post. But it only seems to appear when you go to reblog something so I can’t know if any of my posts are flagged unless I reblog them or someone tells me…
Having gone through this so far, I had a post of mine become flagged, and there was an option to have it reviewed. I selected the option to have it reviewed, and then the flag was removed a couple hours later.

I imagine there is going to be a lot of this going on the next two weeks as tumblr sorts this out.