How do I address a transgender or gender fluid officer?

Morning Lieutenant, CPT, Maj. Just use their rank and drive on.

My main question would be how are you walking by an officer that you KNOW is transgendered, but you don't know how to address them? I would never assume they're trans due to how they look, and if you've been told by someone they are trans they should have also told you how to address them. Other than that if it looks like a sir call him sir, if it looks like a maam call her maam. They will correct you if you're wrong.

In all seriousness though...I'm guessing this isn't a "common" problem, and that the odds are against you having to confront it often. However, if I was six paces from a senior officer of indeterminate gender, and really that concerned, I'd probably hazard popping a salute and calling them by their rank; "Good morning Colonel"...etc...etc. At the least, they can't ding you for failing to show respect, and at worst, you can probably find out pretty fast just what "they" want to be called.

Are they wearing the male uniform or the female uniform... males and females still have 2 different uniform standards, so it should be painfully obvious how you address someone.

Sorry, but this strikes me as a complete bullshit hypothetical to express dissatisfaction with the Army policy. Are you really afraid that the UCMJ will be changed to create the offense of misgendering? Really, you sit around afraid that it might happen, and that you will fall victim to it? Bullshit. You address someone based upon their appearance. Believe it or not, there are already straight men and women who have features that caused them to be mistaken for the opposite sex. It happens. It's awkward and you move on. But if you happened to mistakenly call a female. "Sir" or vice versa, no one has ever suggested that become a UCMJ offense. So why are you so "afraid" that it will become a crime in the future. Again, bullshit.

Any trans officer that reads you the riot act the first time you stutter your way through it is an officer in need of the attention of his or her superiors. Have you never inadvertently saluted a Navy chief? (Some of them have stars that other sm's think are officer insignia), or read a sister service's rank insignia wrong? This is no different. A good officer will correct you politely and you'll move on from there. Also, you could just ask. "Respectfully, Sir/Maam...."

Stand by to see how that officer is addressed would be the safest way, however just use your best judgement and be respectful. If you are incorrect, then apologize in the most non awkward way possible and drive on. That's what I would do in your situation.

Just address by rank to be safe.
In the future we'll probably be asking what their "non-binary pronoun" is. My daughter ran into that with a non-binary TG co-worker.

S'mam.