First, to answer the question as posed, it is probably enough to watch the major news and popular literature, along with the popular scientific literature. What is 'hot' will get reported generally, whether it succeeds or fails. If an area is reported in the BILD (or the New York Times) it is probably pretty hot. Even looking at the titles in indexes in journals in your field will give you a sense of it.
But, let me give some caveats about working in hot areas. There are many.
In hot areas there is a lot of competition. Potential advisors will have lots of applicants and can be choosy about who to accept. They may also be overloaded with students and be less able to give the advice you need.
Also, if the area is only recently hot then those who know the most about it might be in the early end of their careers. They know a lot about some things, but less about academia in general, and may be the most busy of all and mostly focused on their own productivity. Established academics who are new to an area might be less knowledgeable about the newest stuff but most able to promote your career.
But perhaps the most important issue is that of parallel research in hot areas. Lots (and lots) of people may be asking the same questions and pursuing the same answers. It is far more likely that you get scooped in a hot area than in one that is more established. And, depending on the institution, getting scooped can potentially set you back to zero.
It is also possible that today's hot area will be ice cold by the time you finish a degree in, say, three years or so in Germany, and potentially much longer in some other places. Again, this might be the result of a lot of activity finding success in a short time, but it might also be caused by economic changes and needs. Robotics is pretty hot at the moment, but who can predict the future needs of those who depend on robots.
Finally, it is probably more productive to follow up on what you already know, whether hot or not. Your professors in your masters can give you guidance and, perhaps, help you find a suitable advisor. You will have an opportunity after you finish your degree and get established to be more choosy about what you want to study.