You can view the issue permissions directly in the Issues list by using the permission viewer.
First, go to your user profile settings, and check the option "Show an icon for issue permission in issue lists" in the issue settings section.
Now, you should be able to see the icons for permissions in the issues list. Note that if the permission is "All", no icon is shown in the list.
Click on an icon to show the permission info popup. This shows which users, user groups and companies that have access to the issue, and also if the access is read-only or not.
Also note that some users may have access to the issue through more than one permission group, such as when a user belongs to a user group and company that is also set as having access.
You can also see them in the tabs on the bottom om the issue if you view the issue.