We can't ever really know when a tweet has been "consumed" (to use Twitter's artless term) without access logs. However, we do have a few pieces of information that may help. Notably, retweets.
Retweets are the currency of Twitter. They're a transaction; I've consumed this tweet and find it valuable enough to pass along to others. And, as retweet-like functionality spreads to Facebook and other networks, they're increasingly becoming the currency of the Internet.