There's no one answer to this question since it can vary depending on what type of Google page you're trying to access the admin for. However, some tips that may be helpful include:
-Checking the page's About or Contact section to see if there's any information listed about how to reach the page's admin
- searching online for the page name + "admin" to see if any relevant results come up
-posting a question in a relevant forum (such as the Google Page Owner's Forum) to see if anyone there knows how to contact the page's admin.