Forgot to mention that looking at the grain of splits and tears along the log can be helpful too. If it’s curly, the split surface will be wavy too.
On super curly logs, you can sometimes see indications through the bark.
Finally, I’ve been told that the curliest of trees will have wavy grain clear out to the twigs. I even have heard stories of people trying to identify curly standing trees by breaking off twigs and inspecting.