You could say "Excuse me" when they have a clear visible cue of you.

This example is going to be triggering for sexual assualt and verbal assault so I am going to just warn everyone.

TRIGGER

A few weeks ago, I went to the store to pick up a few things. I was using the self check-out option, and while I was in line, this couple was looking at me. It made me feel extremely uncomfortable and terrified; as I once was chased out of a store all together will people trying to get a feel of me.

When he spoke (he identified as male since his girlfriend was on the phone saying "yeah, he is right here") he went to me "So, do you have A d**** or a p****? Can I check?" And then began laughing as I physically began panicking and just left the store.

I think an important thing to remember is that a lot of non-binary/trans* folk have been physically or verbally assaulted countless times. If you want to ask this theoretical person a question, your best bet is not to guess but rather to use a gender neutral approach. "Hello, how are you?" "Excuse me", "May I pass?" Or even "What is your name?".