Almost everything I read is a series so this is a frustration of mine too with indie books. You can't really rely on publication date because with self-publishing the date "published" is usually listed as the date that the book is uploaded to the sales venue. Though on Amazon you sometimes get a date and sometimes you don't (do the authors enter that info?).
I find goodreads to be a good source for series order even for indie books and I always add the info myself when I can when I find one that is missing and I know it. Otherwise, I usually have to go to the author's website to try and find it. The readers really shouldn't have to jump through hoops to get this info when it is so simple to put it in the title. It is very common for trad books to have this info on Amazon, but for indie books it is often missing. It is helpful to put it in the description too
