[plt-scheme] how to determine where a thread hangs?

Hi -

what are some approaches to determine where in code the thread execution (or
multiple threads) hangs?

The issue is quite intermittent (but definitely there), so I am having a
hard time to nail it down, and want to see if there is a way to determine
when a procedure do not return (or blocked).

Any thoughts are very appreciated.  Thanks,
yc
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