Currently, when visiting a page with active flags, mods see an overlay on the bottom:
Unfortunately, we don't see these while in the review queues. There's no indication in the queue that a post has flags -- sometimes the flags (especially custom ones) convey extra information on why a post must be closed/reopened/not closed/etc. And since custom flags are dismissed on closing, this can be even more of a problem.
Could we have something prominently indicating that the post has flags in the review queues?