By convention a lot of modules include a
modulename.api.php file in the root folder, which describes the hooks implemented.
They may also implement
hook_hook_info() to programatically describe the hooks to Drupal.
If that file isn't there, have a look at the other documentation/non-code files included. Sometimes you'll find examples elsewhere in the folder structure.
There's nothing enforced, though, so there isn't a generic all-encompassing answer. For a low tech solution you could grep the module folders for patterns like
If you use the grep method, remember to add a conditional for the quote before the module name, it could be single or double.
In case the above causes confusion:
By convention most hooks are prefixed with the module's name, so that method will work 99% of the time, and avoid false-positives when a module invokes hooks for anything other than itself.
There will be the odd instance when it doesn't, but since there's no formally enforced way for a module to define hooks, anything non-manual here is going to be a best effort however you look at it.