Trends can tyrannize; trends are traps. In any creative industry, the fact that others are moving in a certain direction is always proof positive, at least to me, that a new direction is the only direction. —George Lois
The first thought that comes to mind is why do want to keep up with design trends? My hope is that the answer is to avoid them. A trend is “a current style or preference”*, which runs counter to the goal of many design solutions. An identity, for example, should not be built to be trendy, it may be by happenstance or ignorance, but not by aspiration.
I would carefully evaluate the needs of the problem and follow trends only when it is specifically called for, which would not be often, something transitory or satirical perhaps. If the push for trendiness is client driven, another issue entirely, it may be unavoidable but should be earnestly cautioned against; with case studies if necessary.
To answer the original question, there are two main paths. Look at a lot of design and decide for yourself what is popular and trending or find pundits who will tell you what is or will be trending. For the first, sites like Reddit or Graphic Design Forum are good resources. Search in the graphic design subreddit (r/graphic_design) for “design inspiration” and you can peruse oodles of good choices. If you prefer lists and the opinions of others, it is as simple as searching for “graphic design trends” in your engine of choice. The first return I found was for that was “8 New Graphic Design Trends That Will Take Over 2017” which as the prescient Mr. Lois stated, looks like a list of things to avoid.