If you have a metric ruler, preferably a steel rule, you can measure the fork's crown race diameter with a piece of paper.
Cut a strip about 3-4mm wide and a few inches long. Wrap it around the crown once and mark a straight line across the point of overlap. Unwild and measure, then divide by 3.14. Since the fork is either 26.4 or 27mm the difference in circumference is shy of 2mm, and you should have a pretty positive result. Do it 2 or 3 times to be absolutely sure.
BTW- Aftermarket JIS forks are very rare, and the last bikes to come to the USA with JIS headsets were Specialized and that ended over 10 years ago, so of you have an idea of the age of the fork you might have some confidence whether it ISO or not.
Side note, as one of the last defenders of vernier calipers, I'll add - no batteries needed - to my list of virtues.
__________________
FB
Chain-L site
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“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
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