Test if a list item exists

I have two lists, imaginatively List1 and List2.

List 2 is the master list the rest of the process works with and is essentially a copy of items from List2,

Lets say List1Identifier is the value that links the two lists.

List1 is newly created items.

When an item is created or updated in list2 I want to check if it the related item exists in List1 and if not create a new item. I've read a some wild and varied methods, none of which seem elegant or successful.

Question, how can I check if a list item exists and take branching action on success/fail of that test?

My bad, this relates to WF2010, not 2013 sorry.