Besides the book's cover you can look:
- inside the book for a reading order listing of all books in the series
- inside the most recently published book for a reading order listing of books in the series
- Wikipedia
- Amazon, Barnes & Noble and other booksellers
- Author websites, fan websites or wikis
There is no way to know exactly which source is going to have the information you need. Nor is there any means of judging the accuracy or bias of the information presented on any of the sites (for instance, series numbering on such sites could be chronological
or publication order).