mik5,
One time I was offered something at work that was cholov stam, and the lady showed me the OU-D and said, ‘See? It’s kosher. You can eat it.” How does one explain himself out of such a situation? Should I claim to be lactose intolerant? But on a different day they will see me eating my cholov Yisroel yogurt!
It doesn’t have to be complicated; just be super nice and show gratitude. Smile, and say, “oh, no thank you, but I really appreciate how thoughtful you are for finding this!” If she went through a lot of trouble to get it, or she’s likely to do it again, then you probably should politely and concisely explain your kashrus standards.
If one just said the words Baruch ata Hashem Elokeinu Melech Haolam (e.g., of asher yatzar) and there is a goy who asks you a question, what should you do? (It happened to me one time but I had only said Baruch ata Hashem so I quickly ended off with lamdeini chukecha.)
Smile and hold up a finger, finish the bracha, then apologize and explain that you were making a blessing. If they ask what the blessing was about, you can say that Jews make many blessings throughout the day for various reasons, and that this one was thanking G-d for your body and health.