First off, if you only want your webhook to fire when a specific field is updated you can use field level webhooks. That can often make things easier. See the "Apps" section here: https://developers.podio.com/doc/hooks
With that out of the way: You just use the normal API operations. Since you only get the item_id you probably want to use Get Item or Get Item Basic before you do anything else. Either will contain information on the current revision and you can use that to compare revisions.
All the best,