Determining which versions of the iPhone SDK are installed

Yesterday I was trying to determine which version(s) of the iPhone SDK I had
installed so I could decide whether to download the newest version of the SDK.
No sense in downloading it if I already had it, especially since it’s a 2.4G
download.

Here’s how to do it:

ls /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/  

[Update: I asked this question on Stack Overflow, and this answer gives a good explanation of how “SDK” is
used in the iPhone community.]