The ad auctioning process in FLEDGE is an expensive process. Fenced frames have restrictions for what conditions need to be in place for one to load. If a fenced frame will not be able to load in the current context, it doesn't make sense for ad auctioning to take place only for the ad to not be able to be displayed in the page.
The API could exist as a static and synchronous method of the HTMLFencedFrameElement class, which would check if adding a new fenced frame inside the current execution context's frame would be allowed or not. It would be invoked by calling:
HTMLFencedFrameElement.canLoadOpaqueURL();
which would return either true or false.

See the design document for implementation details.