I have a simple test to predict if someone will be returning to WoW or not: what they do with their mail.
If you're like me, you use the mailbox as an extended bank. 30 days of storage for only a few copper per stack is a pretty good deal. Sorry, make that 60 days. Either way, mailboxes aren't just for transportation, they're also for storage. Like those warehouses out in the distant suburbs with names consisting of mixes of stor, lock, and u.
If I leave for too long, a lot of mail may be destroyed, taking with it huge stores of inks, glyphs, and enchanting mats in addition to a mix of other junk which may add up to thousands of gold. While a thousand gold isn't what it once was, it's still not something to let vanish.
The few times that I've 'quit', I've sorted out my mail, found alts to hold it, vendored some items, mailed some to friends. This indicates that I care what happens. I don't have much use for those items and gold, unless I have some expectation that I am coming back.
An even stronger indicator is if the person uses the auction house to use up items. They might not stick around for the completion, but it does show that they care to take that extra time and effort not merely to maintain resources but to further increase them. And since AH mail deletes after 30 days, they are either waiting for AH results or they're planning to be back in 30 days at most. This person is on a vacation.
At the opposite end is the person who simply quits. They don't take time to save anything because it has no value. Someone who does not care about the game will clearly not care about items in the game. Of course this could merely be a temporary rage-quit But someone who calmly decides to quit and ditches the mail, they're gone.
For the record, my mail is all very nicely stored on the appropriate alts. I sorted it all out before I left.