Whats exactly stopping you from not joining a stacked game? I think those two teammates who left atleast saw the sense and gave the stacked team exactly what they deserved- a game against AI players . You should have just quit the game.
Not all games are stacked. Eventually you can kinda sniff out those that are..
Heres some tips I use..
Those 8 player games with have 4/8 or 6player with 3/6 . Watch them. If it remains open for awhile and those 4 sit together throughout then its probably a stacked game where they are waiting for 4 unsuspecting randoms and will keep the game open for longer then usual because the host really wants to play with his 3 pre-arranged partners.
If you find the top few players chatting as if they know each other well , thats a big sign.
In many stacked games, it feels as if the top 4 players are ALL hosts. Forexample , the second player will ask you to sort your teams out , or the third player will conducting the rules or telling people to ready up.
The biggest tip is to simply ask if you can join the best player and tell them your a newb and it would probably be good for balance. If they boot you then that just saves you from pressing the leave button.
Watch out for Ginius / badmofo (ahahahaha)
Read the name of the game. If it says "skilled only" then BEWARE. It might not be pre-arranged but often what happens is that if the host is satisfied that his side has enough skilled only , hes not fussed with having one or two noobs on the other side to fill up the game.
Simply ask the host if its stacked. Some will probably say yea. Sometimes expierenced people are just fed up with partnering noobs and just want an easy win for a quick rush. They dont want balance whiners to join , they just want 4 players who dont care and just want to play. Just leave the game.
Thats from the top of my head , but with those in mind , Ive avoided stacked teams quite well.