I found plenty of info on this for Safari 3, but things have evidently changed in version 4. I just happened to stumble upon the answer by experimenting, so posting it here
Ctrl + "," to open Safari Preferences > Advanced > “Show Develop menu in menu bar. Close and press Alt to see result.
For version 4, open Safari from the command prompt with the switch: /enableInternalDebugMenu
e.g. "C:\Program Files\Safari\Safari.exe" /enableInternalDebugMenu
For version 3, the switch is: /enableDebugMenu
Note: You only need to do this once -- Safari will remember the setting.
Enable the debugging menu in safari 4 in Windows 7 http://madhukaudantha.blogspot.com/2011/02/enable-debugging-menu-in-safari-4-in.html
Checked in Safari 5.1.7 for windows. In the top right corner click Settings gear and navigate to Preferences>Advanced Tab. Check "Show Develop menu in menu bar"