I honestly have wrestled with this question since I started thinking application of the Book of Revelation. It is not an easy question for most of the book as to what parts of the book are still absolutely predictions of future events. The problem as I have stated before is that the symbols could be more benign relatively speaking than this grand destruction of the whole world as we know it. To me it makes more sense if we look at it from the shaking and judgment on the Roman Empire than on our modern world. I think there is a lot fulfilled or never actually took place because things changed as far as people’s response to the gospel. Prophecy to me is propositional so if the conditions change then the prophecy also changes or does not take place. That said, there are a few things is Revelation that are about the future because of the nature of what they are talking about.
In simplest terms this involves everything in the book from the Return of Christ in chapter 19 onward to the end of the book. I think this is a safe assumption to make because all the events in this part are complimented by teaching by Christ and the apostles. The return of Christ, final judgment and the new heavens and the new earth all are spoken of before. What we see in Revelation is the visual images of what this might look like. I say might because once again this is how it would be at the time of the writing of the book of Revelation and 1900 years have passed since then so they might be even more wonder and glory.
The main issue though is that Christ returns, the martyrs and Christ rule for 1000 years while Satan is bound, the final battles take place, the enemies of God are destroyed, the people of God enjoy the new life in the New Heavens and New Earth. The thing about all these events is that there is still a maintenance of uncertainty about when they could take place. The early church expected Christ return in their lifetime and so notions that Christ would return quickly were everywhere. The problem then is any prophecy that requires certain very specific things to happen before Christ’s return undercut this any time expectancy. That is why I think the seven ages of the church thing based on chapters 2-3 is bunk. It would not be proper for God to reveal for us to always be ready for his return, if he also knew absolutely knew that the church had to go through seven ages first. It would make God a liar.
That is why other than the last few chapters of Revelation I think it is very debatable whether or not the various visions in the rest of the book or Revelation or ‘fulfilled’ or ‘unfulfilled’. It really is impossible to tell sometimes because of our lack of knowledge of what the seven churches would have understood. If we did understand things the way they did, I imagine we could see the connections to events already taken place and the understanding of the political and religious forces of their day.