Status of kinesis stream reader

How do I tell what percentage of the data in a Kinesis stream a reader has already processed? I know each reader has a per-shard checkpoint sequence number, and I can also get the StartingSequenceNumber of each shard from describe-stream, however, I don't know how far along in my data the reader currently is (I don't know the latest sequence number of the shard).

I was thinking of getting a LATEST iterator for each shard and getting the last record's sequence number, however that doesn't seem to work if there's no new data since I got the LATEST iterator.

Any ideas or tools for doing this out there?

Thanks!