Recently I needed to find all hooks exposed by a module.
Most drupal modules do not document it, or the documentation is not up to date.
So to do? Search for pleases where module_invoke_all is called. This will give you the answer.
fgrep --exclude='.svn' -r module_invoke_all <module-directory>
I researched the Membership suite set of modules and got the following results:
module_invoke_all('ms_membership_signup', $account, $purchase, $plan);
module_invoke_all('ms_membership_signup', $account, $purchase, $plan);
module_invoke_all('ms_membership_refund', $account, $plan, $order, $payment);
module_invoke_all('ms_membership_signup', $account, $purchase, $plan);
module_invoke_all('ms_membership_renewal', $account, $purchase, $plan);
module_invoke_all('ms_membership_modification', $account, $purchase, $new_plan, $old_plan);
module_invoke_all('ms_membership_expiring_soon', $account, $purchase, $plan);
module_invoke_all('ms_membership_expiring', $account, $purchase, $plan);
module_invoke_all('ms_membership_cancel', $account, $purchase, $plan);
module_invoke_all('ms_membership_payment', $account, $purchase, $plan);
Of course you can also find the hook through the module's documentation. But that documentation does not always exist...