This series is much more difficult to attribute than US coins, especially if you're a beginner. This interactive guide will help you attribute a piece, and if you're not sure, you can always reach out and I'll be happy to help.
Start with the first question, and click on the correct metal. The page will automatically take you to the next relevant question until you find the right type. I've only used size/weight when it's the only option; I know that information isn't always available if you're shopping online, but sometimes it's the only way. These numbers are approximate - select whichever is closer to your coin. There's a lot of variation within every issue.
This also assumes you're not able to read the denomination. If you already know it's a gold nisshu and just need to know which type, this will be a bit tedious. I recommend selecting gold, then searching that page for "nisshu" (ctrl+F on a Windows computer will let you search).
Note that only the first question will take you to an entirely new page. All of the following questions for each metal are on one page, so you can go through questions quickly without have to wait for each to load.
If you want to download an attribution guide, there's a basic spreadsheet available below. It doesn't have any images, but it helps if you're already familiar with the big differences and just need help with the minutia. I'll eventually spruce this file up to be prettier and more user friendly, but it functions!