If an audiobook is a part of a series it will have a series icon (a stack of papers) under the book cover.
There are several ways to get to the series page. You can go to the book page and then tap the series name under the book title. Alternatively, you can tap "More" under the cover to access the book menu and navigate to the series from there.
You can also get to the book menu without first opening the book page by long pressing a book cover anywhere in the app. In the menu, you can tap "Go to series" to access the series page directly. Now you will be able to see which audiobooks are a part of the series and what order they come in.
Now that you have found the series page, you can even choose to follow a series you like. Up top you can tap "Follow" to start following the series. You will now get a notification on your phone if a book is added to the series. You can always find all the series you are following under "Home".
To receive these series updates, you will have to allow notifications from BookBeat. You can do this under "Profile". Read more about notifications here.