How do I tell which version of monotouch I have installed?
There's several ways to get the version number.
From the terminal you can do:
$ /Developer/MonoTouch/usr/bin/mtouch --version
mtouch 5.3.5.1340204820
or
$ cat /Developer/MonoTouch/Version
5.3.5
Also from MonoDevelop you can do About MonoDevelop then select Version Information and this will give you (a lot of data, including MonoTouch version)
...
Monotouch: 5.3.5
...