Steps to make sure you can answer this question with yes.

Create a master login that has administrator access that will allow you to log into each account.

Make sure you can remove the login access of the current person managing the accounts, just in case your paid search manager fell of the Earth on bad terms.

Have your paid search manager create a master list of links and logins for all current accounts across all engines or directories.

Have the updated list sent to you on a regular basis and save it in a safe place that cannot be easily accessed.

Make sure you are familiar with logging into each engine or directory and the navigation of how to change passwords. Just in case you need to change them in a hurry.

Have your paid search manager send you copies of any updated custom reports and a quick update of reasoning behind recent changes applied. This will allow you to have a seamless transition to a new person in the eyes of the clients.