To be honest, I don't think people can tell each other's ethnicities apart as well as we think we can, and/or the differences aren't always as great as we think they are. Sometimes you can easily tell and sometimes you can't, and sometimes when one person can easily tell, another can't. Sometimes you can tell only because you know -- the old "Now that you say that, I can totally see it!" syndrome. Speaking of casting a Filipino as Thai, I'm white and Filipino, and I've had Thai people think I was Thai. (I've also had Hispanic people think I'm Hispanic.) (And I've been mistaken for Native American millions of times, so Lou Diamond Phillips and I should get together to play a Native American family.) I teach international students from all over the world, and they are sometimes surprised to learn classmates are from the same country as them (i.e., the same ethnicity) and they're sometimes surprised to learn classmates are not.
I don't want to say that all Asian people are interchangeable, because I don't think that's true. But I also think enough variation exists within groups that like...my problem with Amy Hill playing a Filipino on Crazy Ex-Girlfriend is with the fact that there aren't enough Filipino actresses in Hollywood (or that they didn't look hard enough to find one), not that she doesn't conceivably look Filipino.