Is there any way to see the actually capacity used of the SSD in a fusion setup? I'm curious to see how much it is actually pushing off to the HD. I will have my new mini with 240GB SSD and 1TB HD tomorrow. I have a 160GB iPhoto library and around a 80GB iTunes library. I do not want to waste the capacity of the SSD with those items, so I'm considering just managing where I store items rather than use Fusion.
No, but it's unnecessary in a Fusion Drive setup. Worrying about how full the SSD is with data is "old" thinking. The OS will start by filling the SSD first. Until you have enough data to overflow the SSD, all of your data will be there and will be quick to access. Once the SSD is full, all new data (up to 4GB) will be written to the SSD first, then will go to the HDD. Once that's done, it will use idle time to determine which data on the HDD would be best served by moving up to the SSD. And it's done at the block level, so it might not even be all of a file, but might just be, say, the first half of it, while the rest is simultaneously read from the HDD.
Really, you just need to get past the idea that you somehow need to manage which data is where. IF your SSD is full, and if you use any individual piece of data regularly, OS X will move it to the SSD. If not, it will be on the HDD. This means that perhaps frequently viewed photos or songs might end up on the SSD, while the rest is on the HDD. Or maybe only the first few seconds of a song will be there, while the rest streams from the drive. The OS will figure out what works best. It's entirely possible, maybe even likely, that your photo and iTunes libraries might be broken up across both drives. They don't have to live in any one place. They live on both, as needed.