One thing that comes into our mind when the outcome or the result of the game is too different from what we expect is that it was a fixed game which is just our assumption. Some games only turned out different simply because sports are sports and anything can happen anytime. The result won't always go in favor of what we want.
Exactly, most of the time we're just on the assumption stage considering we vouch for the player/s or for the whole team.
If you are a watchful person, I believe you can suspect the behavior and expressions of players, coaches and support team, but it's impossible to be assured without further investigations and evidence which must come from someone who is inside the industry, and not from a random spectator. In every sports there are cases of suspicious results people didn't accept by nowadays, although they can't prove anything with certainty at same time.
What gamblers can do is to avoid minor leagues in order to decrease the chance of betting in a fixed match and being "scammed".
Probably this are for the keen eyes and can literally see the behavior and expression of the individual, and you can tell I can't do that. The thing is, most rigged games can only be known or catch is when the game already happened even with just tiny details. And the worst in it, bettors already win and even if you protest, that would take a lot of time or worst as I'd say, it will never be changed. Take an example are some boxing matches.