The quiz website 助数詞を練習 (Counters Practice) is a new tool you can use to test your counters knowledge. Finally, you can easily practice counters in batches so when you go out into the real world, it all flows more naturally!
To start, there are four study packs to choose from: N5 Counters, N4 Counters, Essential, and Common. Pick the groups you want to study, and click "Start Quiz!" From here, you'll see English like "19 coworkers" or "5 straws," and it's up to you to type the correct number + counter combination in hiragana. Hit enter, and you'll see a result along with information about the counter combo.
You have a few cheat-y options during the quizzes too: if you encounter a question you don't want to answer, click "Skip this question" to remove it from the test. When you make an honest typo, click "Ignore Answer" to have it removed from the results. These are fine, but make sure you're being honest so you can accurately assess your ability at the end.
If you're having trouble with the quizzes or want to study counters you've never seen before, click the "Explore" tab on the home screen to choose counters you want to study. You'll get a grid displaying counters information which you can use to beef up your knowledge before your next quiz session.
The website is still in beta, so there is room to improve. For example, it would be nice to have the ability to make custom batches of counters, because the current ones are a bit arbitrary. (What's the difference between "essential" and "common" anyway?) Also, quiz questions like "39 o'clock" sometimes appear, so the algorithm could be tweaked in the future to remove these kinds of possibilities.
As it stands, 助数詞を練習 is an excellent way to test your counters knowledge—something that's not offered many other places, at least not at this level of quality. Hopefully more counters and features will be added in the future, and 助数詞を練習 will become the place to practice counters online!