1) The Chrome extension Dex logo in chrome could change color (or have a little notifications sign on it) if the visited linkedin profile is already saved in the Dex DB.

2) The Dex Logo (right on the name on the profile page - could have a little notification.

3) Alternatively - a small Dex icon for each LinkedIn profile link on the page; with notification if already saved in Dex. Here's an example of the 3 points above functionality for HubSpot - https://chrome.google.com/webstore/detail/free-linkedin-to-hubspot/gmlnodfledbnihglacaejlfdjimaapkk