Rails 3: How to identify after_commit action in observers? (create/update/destroy)

I think transaction_include_action? is what you are after. It gives a reliable indication of the specific transaction in process (verified in 3.0.8).

Formally, it determines if a transaction included an action for :create, :update, or :destroy. Used in filtering callbacks.

class Item < ActiveRecord::Base
  after_commit lambda {    
    Rails.logger.info "transaction_include_action?(:create): #{transaction_include_action?(:create)}"
    Rails.logger.info "transaction_include_action?(:destroy): #{transaction_include_action?(:destroy)}"
    Rails.logger.info "transaction_include_action?(:update): #{transaction_include_action?(:update)}"
  }
end

Also of interest may be transaction_record_state which can be used to determine if a record was created or destroyed in a transaction. State should be one of :new_record or :destroyed.

Update for Rails 4

For those seeking to solve the problem in Rails 4, this method is now deprecated, you should use transaction_include_any_action? which accepts an array of actions.

Usage Example:

transaction_include_any_action?([:create])