The mt command does not show the "write protect" status of a tape. This would presumably apply to other devices that support write protection, but I cannot test for that.
You cannot write to write protected tapes (see BID9933), and get an error if you try - but it is very hard to find the error message if you tar to a write protected tape using a remote login, and if you use a changer remotely, this is seriously hard to debug!
So two bugs (possibly feature requests):
1) mt -f /dev/sa0 status
# Should report "Write protected" or "Write enabled" as appropriate
# I tried to implement this myself, but I think it might be easier for someone else to do it!
2) tar c -f /dev/sa0 file
# should write "Error: target is Write protected" to stderr
# which presumably requires significant design effort, as both cam and tar
# would need changes.