There's only one way to find out, open it up and take a look and a sniff. If you search the Hotline, you'll find at least one recent discussion about the subject.
I am assuming the whole husk is intact, not the coconuts that have had the husk taken off on the top. I have heard different things - from a year (I am doubtful of that) to four months. The key is that they stay in a cool place. I agree with ChefOno, the only way to know is opening up and taking a sniff. You will know immediately if it is moldy.
Totally agree with the other answers. There's no real way of knowing until you open it up. If the meat is no good, you can always turn the shell into a coconut bowl. See @coconutbowls on instagram for a what I mean. They have a website too www.coconutbowls.com
I have little culinary experience of coconuts, but there's a reason why they grow nearly everywhere near the ocean that's warm enough- coconuts are able to survive extended periods floating about in the ocean and still be viable as seeds when they arrive- they are very durable.
Susan W is a trusted source on General Cooking.
The smell test doesn't cover all things spoiled. Here's what Still Tasty says.
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Well played. You deserve a cookie.
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