What you do is look at the rationale that was given before it happened. You look at their reasoning and figure out if it was actually what happened in reality. Just make sure you are not picking up on their rationalisation after the fact.
The general problem is each has a mental (or computer) model of the world that is a vast simplification of the world. When they get it right it sounds like they understood and accounted for all the variables. They never did.
Being right means little when there are 100s of folks with predictions and reasons.
Sometimes you just need to trust people on certain things to be able to make useful decisions having very little or no knowledge in the subject.
One useful way of dealing with this is to look at the past performance of the person.
Assuming you have no other information about the market, if you see a person having track record of well-reasoned, accurate market predictions it is probably as good signal as it gets that you should trust their predictions.
Ideally you would want to check this with other people having knowledge in the topic (and also good track record), check that their knowledge is still applicable to circumstances (if they were able to give good predictions in peace maybe they are not suited to doing this in the time of war) and hopefully also educate yourself just a bit to be able to ask clarifying questions and spot obvious problems.