"Well," I said, "what do you mean by 'good' English?"
"Good!" he shouted. "You know, good, as in...good?"
"You need a better description than 'good'," I said. "Everyone thinks they speak 'good' English, the same way they think they make a 'good' tortilla española, or that their mother is a 'good' person."
The problem with words like "good" (as I've said before) is that it's an extremely useless and inaccurate word. That's a problem for managers, who may need employees with specific English skills, or for job candidates, who want to show how much better their English is. For more people, however, that's a problem. If they haven't done any kind of English exam, such as the First or TOEFL, how can they show what they can do?
Here's one way. If, in the past few years, you've had to use an English textbook, take a look at the back. There should be a code listed on the back with a letter (A, B or C) and a number (either 1 or 2.) That code will tell you where you are, more or less, on the European Common Framework of Languages - the EU system of showing where students are, depending on what they can do.
This description, from the Oxford University Press, gives a pretty basic background into the history of the Common Language Framework, and how it works for all languages, not just English. (So if you're also studying French, German, Italian or any EU language, there'll probably be a similar code on your materials.) To make the description even more basic, here's a quick summary.
If you're A1, you're a total beginner. You had little or no previous knowledge of the language before you began taking classes.
If you're A2, you're more than a beginner. You can talk about yourself, you can ask for basic products and services and you can have simple conversations.
If you're A2/B1, you can speak more or less clearly about yourself, but having a detailed conversation may give you trouble. You might have one specific skill, such as listening or writing, that gives you trouble. If you're in Spain, you know that you have enough language, but you know that you don't have enough English to survive in an English-speaking country
If you're B2, your skills are advanced enough that you could survive in an English-speaking job, or with an English-speaking family, without too much trouble. You might not have enough language to do anything really complicated, like negotiate, but you could present an opinion about a topic, solve a problem with a friend or co-worker, and listen to a short lecture or radio show without too much trouble. (This is the level of competence that you need to pass the First Certificate.)
If you're functioning at the C1 level, you're doing pretty good! You've probably spent a lot of time with native English speakers, or you've lived in an English-speaking country. Watching movies or reading books might take you longer than it would for an average native English speaker, but you're comfortable in most social situations; you have enough language to find a solution to a detailed problem, and you could probably read a longer magazine article or take part in a university class without too much trouble.
If you've worked your way up to the C2 level...congratulations! You're practically the same as a native speaker of English! Now, no one's going to confuse you with a native speaker, but you can probably do most of the things that a native speaker could do without too much trouble.
Obviously, this is a HUGE simplification of the system, and if you'd like a more detailed idea of what people can do at each level, the Association of Language Teachers of Europe Can-Do Document has a much better description than the one I've provided here.
So give yourself a grade -- it's much better than simply saying "I'm good"!