From the command line option I can not see that there is a way to see what bfg will do before doing it.
If I run this command:
$ bfg --strip-blobs-bigger-than 1M --replace-text banned.txt repo.git
Can I get a list of files larger than 1M before actual deleting them?
See https://github.com/rtyley/bfg-repo-cleaner/issues/17 for a discussion on a dry-run feature.
Essense of my comments there: since Git makes it super-cheap to make additional local clones where you can perform test-runs of BFG, and this provides you with real output you can verify in addition to a report, that is superior to having a report-only or dry-run mode.
Just make an additional local clone of the repo, run BFG, then read the reports produced and examine the repo.