By your preferred definition, there may be very few "good" foreign teachers in Taiwan. Ask your boss what a good teacher is to him/her. Really.
People want different things in difference circumstances. It's relatively easy to know if you're a good plumber. Basically, people want water to flow into and out of their homes in a clean and sanitary way. If you've fixed their broken pipes, or fixed their flooded toilet, and especially if you've done so in a charming manner that puts people at ease, and without showing too much crack, you're a good plumber.
It's never that way with teaching. Here, some actually want to learn. Some bosses actually care how, what, and if, you teach. They care about your own sense of efficacy. But they're RARE as hen's teeth. Mostly, they want someone, as stated earlier, who is a good enough entertainer that kids/adults will want to keep paying for spending class time with you.
Even public schools here are the same. I worked for one which was, by all accounts, a very good one, and who's interview process was very tedious. But what they wanted was that each kid (about 40 usually) in a forty minutes class should speak English as much as possible. THEY DID NOT EXPECT THEM TO GAIN ANY KNOWLEDGE WHATSOEVER ABOUT ENGLISH OR ANYTHING ELSE. Really. Just make every kid say as much as possible in English. We know they won't learn anything.
Back "home," it's the same. The story is still that effective teachers are wanted and needed, but the definition of "effective" has changed. It used to be that teachers in the States were supposed to teach you how to read, how to learn, how to think for yourself so that you could figure out your curriculum and make better life choices when you left school.
Now, we're supposed to produce results on a standardized test. This is where the money is, and this is all that matters. The kids don't. It does NOT matter that the people taking the test may have no personal stake in the outcome, may have been up all night, may be hungry, may be stressed out, may just have transferred into the district, may have an IQ of less than 80, may not speak the language of the test, almost positively don't give toss, are bored by the entire process, are thinking of what's for lunch---whatever! Many districts, even entire states, are publishing a teacher's class average test results in newspapers and on the nightly news.
Parents can ask to have a child moved to a different teacher's class, in some districts, if the average isn't high enough. Raises can be withheld from low scoring teachers, schools and districts that don't perform lose money, and can be taken over completely by outside managers. Teachers lose jobs, are publicly humiliated, and scapegoated for the entire fiasco. So . . . guess how much quality instruction usually goes on in a typical classroom? So what is a good teacher?
That's right. And my own, personal, problem is that I just care too much. But what are you gonna do?