Red flag 1: too many "key characters"
Blizzard, as a rule, does not aim to stuff x-packs full of characters that ought to be there. Wrathion did not play a role in Legion despite being heavily involved with preparations for the third invasion in MoP. Odyn does not play role in BfA despite it being his direct duty to protect Azeroth. Do not expect the next expansion to have the same cast as BfA. At best Sylvanas will play a major to minor role while the rest of BfA characters will be relegated to being supporting cast. The main cast will predominantly be completely new faces or older characters with a facelift (see: revamped Saurfang and Nathanos in BfA, revamped Illidan in Legion etc.)
Red flag 2: the story is too detailed
When a new x-pack is revealed, the story is still in the storyboarding phase. At best you will get a premise but that's it. So a realistic story leak would look more like "Sylvanas has opened a rift into the Shadowlands" rather than "Sylvanas has opened a rift into the Shadowlands using Xal'atath with N'Zoth trapped inside in order to complete her bargain with Helya which somehow ties into the burning of Teldrassil, the Lich King, Bwonsamdi's boss, Anduin, Il'gynoth's whispers and God knows what else".
Red flag 3: the name is lame
C'mon, Blizzard actually has professionals working on this stuff. "Sisters of Sorrow" sounds like complete ass and is not catchy at all. As a rule, consider this: does the name of the x-pack make sense to non-WoW players? Is it likely to entice them to try WoW? If the answer is no, the leak is fake. Exception: Mists of Pandaria (Pandaria only makes sense to WoW players but since it's a derivative of "panda", it was easy for non-WoW players to infer the theme of the x-pack).
Red flag 4: it's vague / riddle-y
Looking at you, Chinese "leaks". If a leak can be retroactively interpreted to fit several things, it's as fake as fake gets.
Red flag 5: the features are too detailed
No, Blizzard hasn't decided what kind of ilvl the newest x-pack feature will reward. Neither have they decided on 10 new allied races in advance. Compare: "a new feature will be artifacts with progression systems reminiscent of talent trees" vs. "a new feature will be <generic artifact x> which will... <detailed breakdown of how exactly the new feature will work including full lists of class traits / abilities>".
Red flag 6: it reeks of fanfiction
NO. HIGH ELVES WILL NOT BE THE NEW ALLIED RACE. TINKERS ARE NOT GOING TO BE THE NEW CLASS. THE NEW EXPANSION WILL NOT BE INSPIRED BY VANILLA.
Red flag 7: the leaker does not sound convincing
If you have to justify your leak and explain why / how it makes sense, why not come up with a more convincing leak in the first place?
Feel free to add your own red flags