Rules Condition - New node or existing node when building form

Hi,

I'm looking for a bit of help with a rule I'm trying to create.

When form is being built I want the rule to see if the user is creating a new node page or whether they are editing an existing node page. If it's new content then all fields are available to edit but if they are editing an existing node then I would like to disable some fields.

I've tried various ways (from checking if published checkbox is ticked to executing PHP code) but nothing seems to be working at the moment.

Could anyone help with the condition I need?

Nathan