View all Growl notifications
It's not a built-in feature so you won't be able to view past logs.  However, you used to be able to enable/disable logging of notifications with scripts I've included below but I don't know if these still work.  If they do, then you could enable logging of future notifications.



Enable Growl Notification Logging:  

#!/bin/zsh -f

function growl_setdefault { defaults write com.growl.growlhelperapp $* }

growl_setdefault 'Custom log history 1' "$HOME/Library/Logs/Growl.log"
growl_setdefault 'GrowlLogType' -int 1
growl_setdefault 'GrowlLoggingEnabled' -bool YES  




Disable Growl Notification Logging:  

#!/bin/zsh -f

function growl_setdefault { defaults write com.growl.growlhelperapp $* }

growl_setdefault 'GrowlLoggingEnabled' -bool NO