Strange issue with Fusion Drive - how to tell if it's working?!

Well,

I do not have Fusion. However, if you open up disk utility, do you see 1 Mac HD, or do you see separate drives?
If it feels slow, and you have doubts, maybe you could opt to try to reformat and install as new.

however this is what its supposed to look like if it was working:
http://farm9.staticflickr.com/8052/8148539772_98d9c5727c_b.jpg

Go into System Info and look at the SATA data. If you select the HDD you should see the following bold headings in the data pane.

EFI: which is disk1s1 and contains EFI
disk1s2: which contains corestorage
Recovery HD:

The SSD should show the following:

EFI: disk0s1 which contains EFI
disk0s2: corestorage
Boot OS X: disk0s3 which contains Apple_Boot

As you can see, if Fusion is set up properly, the SSD is the boot disk. It's good to see that Apple assigned the recovery disk to the HDD.

The two drives are linked logically, so the Disk Utility shows them as a single unit.

You say the HDD is 90% full. Your SSD could also be nearly full, which could slow down its operation if there's a lot of swapping between it and the HDD. I don't know if there are any utilities that could show the usage date for each individual disk. DaisyDisk has two versions, and the non Appstore version has more capabilities that Apple didn't allow in the Appstore version.