CRM 2013: Differentiate between create and update in a WFA plugin (C#)

I have created a plugin (C# code dll) which I am using as step of a custom workflow. I want this workflow to kick in the case both creation and update of a certain field.

Now, I am wondering how in my C# code

protectedoverridevoid Execute(CodeActivityContext executionContext)

I could distinguish the new creation of a record and update of an existing record