It's very much real, you cannot tell the difference between an upset and a fixed game, they will try to make it real because there's a big risk for them if they are caught.
If you're on the expert level of analysis for the sport that you love and are a fan of. You'll be able somehow to spot on which is a true upset and likely a thrown and sold game.
But it's true that there's difficulty in finding which of them are actually thrown because when there are fixed matches, they're likely not to do obvious things that will destroy and spot it by their fans.
Even if you are an expect you can’t know until the start of the match, we can just notice this right after so technically many bettors already placed their bets by that time, and knowing that its a fixed match it can ruin everything. This has been an issue for many boxers and matched, just place a bet that you can afford to lose always.