Without the context of the thread, it's often hard to tell a shitpost from a "normal" short comment.
Sorry but I don't get it, what do you mean? I think it's the same.
In some cases, it's obvious from the post alone that it is a "reportable" shitpost.
In other cases, a very short post might still be a "meaningful" post in reply to the OP.
E.g. posts like:
+1
Good Job!
Nice!
LOL
are certainly not high quality or really useful.
But should you report them?
What if they're the acknowledgement of some good work of the OP?
Therefore, before you report a post, you should always have a look at the thread in which the post has been made.