It depends, really. As a rule of thumb, I would say if you could describe a scene in one word as 'gory' then it's most likely going to be R (which of course means it needs to be approved by a moderator). Minor depictions of violence and gore that isn't that detailed would likely fall into the PG-13 rating, but any well-described gore would almost certainly be R.
One thing you may want to try, if you are unsure, is to attempt to avoid describing the gore directly. Let's say you are describing a war scene. Unless it is vital to the plot, try and have your character focus on other things besides the carnage. You really can be creative about it, whether your character is trying not to think about it or simply isn't paying attention in his concentration to the task at hand. Minor, PG-13 descriptions of it here will often work well, as not only will you avoid having to rate it R but it can also leave a good amount of what is happening to the readers imagination. Certainly this will not work all the time, but you would be surprised how you can pull it of sometimes.