How will Google “Fresh” measure if a page is actually “new”?
Will it be based on indexing or does a newly updated page count? For example if there is a page that has product reviews that are updated regularly, would that count as “fresh”?

By all accounts, it will be determined by initial crawl date. When Google first discovers a page will matter most. Also, RSS discovery and pinging services Google uses like PubSubHubBub are likely to play a part.