Even if you could find out whether a block is processing an item, it wouldn't really help you achieve your goal. That's because you would need to check the state of all the blocks at exactly the same moment, and there is no way to do that.
What I think you need is to somehow manually track how many items have been fully processed and compare that with the total number of items to process.
You should know the number of items to process from the start (it's you who sends them to the buffer block). To track the number of items that have been fully processed, you can add a counter to your parsing action block (don't forget to make the counter thread-safe, since your action block is parallel).
Then, if the counter reaches the total number of items to process, you know that all work is done.