How to determine efficacy of index on child side of foreign key

Given a parent-child relationship between two tables, such as:

table ORDER
    order_id PK (already indexed)
    order_name

table ITEM
    item_id PK (already indexed)
    order_id (FK to ORDER.order_id)
    amount

I want to determine whether there would be any benefit (performance improvement to SELECT queries) in adding an index on ITEM.order_id.

What information should I gather that would help me determine this? (such as structure of the queries my application runs against these tables, etc.) I do know that my application regularly runs queries that perform a simple inner join between these two tables. Are there any general principles that apply to index tuning for this kind of scenario?

I am using Oracle 11g.