The bad news is that historically, the Japanese didn't use a hallmarking system.
After the Showa period, commencing in the 1920s, Japanese silver usually carried a decimal stamp of something such as “950”.
Prior to the that, during the Meiji and Taisho periods much Japanese silver was either unmarked or carried the Kanji for “Pure Silver”. Normally, that meant 1000/1000 purity.
In the Edo period, the item might not be marked as silver at all and the metal was relatively rarely used when compared to Europe.
What does this mean for the collector? Let's assume we're talking about the times before the 1920s.
Look for the Kanji
If it has the stamp for pure silver, that's a good but not necessarily conclusive sign. Use common sense and testing if you're in doubt.
Check the maker's marks/signatures
You'll typically need to translate the Kanji or possibly Hiragana but if you can, you may be able to find the artist or maker's mark recorded somewhere else. If they are and they're known to have worked in silver, then that's another good sign.
Expect to be frustrated here though. Documentation of many makers is scant and particularly so in non-Japanese languages.
Ultimately, if you really wish to be 100% sure, you may have to arrange for a test or ask the vendor if they have done so (though ask for guarantees if they say “yes”).
Don't be misled by statements such as “non-magnetic”. There are many silver-covered alloys and composites that are non-ferrous but also certainly “non-silver” too!
Check the quality
Silver was moderately rare. It was typically only used in high-quality work.
If the piece lacks quality (e.g. a cast without fine definition) then it is unlikely to be made of silver.