Well, you'd know it was high because you'd get along with most, if not all, people and adapt to them and at the same time get those people to adapt to you as well. It requires accurately understanding them on an emotional level, something people with autism are predisposed to struggle with, and something that means you would be capable of not only understanding their emotional needs, but helping to fulfill them, while fulfilling yours at the same time.
It's kind of something you evaluate yourself on, or at least for me that's how it works. Because I imagine an EQ test would have the same problems that IQ tests have and if I then tried to create rules for evaluating it, well, it would be silly and flawed because it's not exactly about rules, but adaptive (fluid?) intelligence. I noticed this about IQ tests before, that they seem centered around testing one's ability to apply certain ways and/or types of thinking, but not testing the person's ability to think about thinking (or metacognition), something which would help someone much more with navigating through life because it would bolster their ability to learn, rather than apply absolutist ways of thinking or understanding about things.