How to determine how used up my ssd is?
Your speed issue isn't likely to be because the SSD has been used too much, as sectors would simply stop working and the space would start to decrease rather than slow down.

The issue is more likely to be caused by TRIM not being enabled in the OS, so the drive is doing it's garbage collection (sometimes referred to as GC) routines as and when you write information. This will cause a considerable slow down as with TRIM enabled it will happen as soon as space isn't required anymore.

If you're running Mountain Lion you can run this script to enable it.

There's an app called TRIM Enabler, but based on feedback across a wide variety of sources, I've always avoided it.

You can check whether TRIM is enabled by clicking on the Apple logo in the top left corner, About This Mac, More info, System Report, and then scroll down to Serial-ATA, and then look on the right pane for TRIM Support yes/no. If you're not patched, then it will say no.