Built from the ground-up to utilise Microsoft's DirectX 12 to its fullest, the 3DMark Time Spy benchmark from Futuremark is the first benchmark specifically designed for the purpose of testing the new graphics API. Finally out of beta, the benchmark is now available to users of 3DMark in Windows 10.
In addition to the basic DirectX 12 features, Time Spy also delves into advanced modes including Asynchronous Compute, Explicit Multi-Adapter and multi-threaded processes. In principle it will be a more rigorous exercise compared to both ad-hoc in-game testing or in-engine canned game benchmarks, and will increase in relevance as more games are released which incorporate these features (such as the recently announced Civilization VI).
Time Spy renders at 2560x1440 (i.e. 1440p), which should give you some indication of the class of GPUs it targets. Much like 3DMark's DirectX 11 FireStrike benchmark, it's likely that additional Time Spy render resolutions will become available in the not too distant future.
3DMark can be found at http://www.futuremark.com/benchmarks/3dmark, or purchased via Steam here for £7.60 until July 23rd. Those who purchased 3DMark prior to July 16th can optionally pick up the 3DMark Time Spy upgrade*, which unlock additional custom benchmark options and a 'skip demo' feature, for £3.49.
* The 3DMark Time Spy upgrade is optional and not required to run the benchmark.