I have a script that I run using osascript from a Run Script Build Phase in Xcode 4. This script does some checks to try and catch any human-error in plists and suchlike.
A quick test on my own system shows no difference in the detailed script output between straight "Build" and "Build and Archive".
Also, you can't test for the presence (or absence) of the archive, since it only gets created at the very end of the process, when all scripts have been run.
As far as I can see, there is no current option within Xcode 3 to do this. Maybe file an enhancement request with Apple?
While you wait on Apple, the only solution I can offer is to use xcodebuild and xcrun as part of a command-line shell script, where you would know if you are archiving or not. This is not a stay-in-Xcode solution, but it does have a lot of flexibility.
Xcode "Build and Archive" from command line