As a more direct answer to your question, while Photoshop's running, look at the free space on the drive.
On my Windows system it makes files that have names like "C:\TEMP\Photoshop Temp4784896316". I don't know if the names are similar on a Mac, but I suspect they may be.
Now and again if Photoshop crashes these files can be left laying around, and have absolutely no use.
-Noel