Fusion Drive: free space on SSD
At least 4 GB of the SSD portion of a Fusion drive is left empty at all times. That space is used for incoming writes. If there's less than 4 GB of empty space on the SSD, Core Storage will start offloading data to the HDD.

So if all your data is less than your SSD capacity less 4 GB, then you have whatever your SSD capacity is (128 GB on all Apple supplied Fusion drives) minus the amount of your data. Otherwise, you've got 4 GB free.

As an aside, the reason df doesn't work in this case is that it only reads filesystems. The filesystem sits atop the logical volume created by Core Storage (the software behind Fusion) — there's no "SSD portion" or "HDD portion", to the filesystem it's all one big drive.