How can I check the progress of a large Assets indexing job? I just uploaded 48,000 photos to Amazon S3, added the bucket to Craft, and am now trying to add them to the Assets library. Where can I see the logs for this?
It's not a log per se, but you can check out the craft_assetindexdata table of your database. That will show you all of the assets which Craft has indexed.

You can see how far Craft has gotten by taking a look at the recordId column - all of the already indexed entries are going to have a value in that column while the records still waiting to be indexed are going to have NULL there.