I just ask the dog, A/S/L and when they don't answer, I look to the owner and ask them to fill in the blanks.

"WHO'S A GOOD DOGGO?" or "Cute pupper!" are my general exclamations upon seeing any dog in the wild.

I usually go with cute adjectives in front of "baby" or "pupper"

I just go with the  neutral “hi little baby” regardless of breed, size, age, gender. the safe approach 

Is it bad that I default all dogs as being "good boys"??? (fwiw I'm a cat owner, not a dog owner)

My cat is misgendered often, and it's amazing how much I get offended on her behalf when it happens. 

But "so what do we have here" is far too open-ended, and more likely than not, you're going to have to have a prepared follow-up. I would say continue asking questions using "they/their" and let the owner eventually slip the proper pronoun into conversation themselves. 

How do you approach without knowing if the new dog is a boy or girl? I always feel SO awkward when I misgender the dog — even though as a dog owner, I never really care — so my new go-to is "Who do we have here???" Safe. Unassuming. And then you get hit with a name that could go either way. Help! 

When meeting a new dog...

Because you love more than just Entertainment.