@cowanon Well thats a hard question to answer without getting into a bit of philosophy.

The first thing we have to understand is that we are dealing with a matter of degrees. Nothing can truly be proven to be true absolutely. But we can give enough evidence that it is reasonable to conclude something is true, at least until stronger counter evidence comes along.

So the goal is not to ensure you are never wrong, but only maximize your chances about beind right, and being aware of how certain we are, which is a bit of a judgement call

So when something is asserted to be true we are really thinking of two things, what is most likely fact, as well as how confident we are.

If we want to assert something is true and be confident about it then we really want to see a few things, ideally. We should find good published papers in peer-reviewed journals that agree with the assertion, we should see a track record of several such publications proving the same fact in different ways, we should see very few, if any, papers that counter it, and we should see the vast majority of scientists tend to agree (this usually goes hand in hand with the papers being published in the first place.

What we should be skeptical of are singular papers, papers that dont pass peer review, etc.

Now what you asked about control groups and and double-blind studies, I will answer that in a second, but keep in mind those are things scientists engaged in peer-review do. Peer review is the process a paper in a well respected journal must go through before being published. It is where other scientists try to point out errors or unreliable practices in papers and challenge the author to correct it, and if they cant it wont get published. One of the things those scientists will look at include if study is double-blind, or if it has control groups (Among other things)

Of course its important you be aware of these things so you can provide your own judgement as you read the paper, but keep in mind you are almost certainly not an expert on the subject you are reading. So in many ways the expertise of the scientists or did the peer review is more valuable than your own judgment at that point.

One thing that is very useful if it exists for the topic you are investigating are meta-studies, those are studies that review all the studies on a subject and try to determine what the consensus is among the studies, they can be very telling.

So with that said a control group is a group representing the natural "background" tendencies. For example if we are talking about cigarettes causing cancer the control group are the people who dont smoke cigarettes so we can see how likely cancer is on its own before we make any assertions as to the effect of cigarettes on it.

Double-blind refers to the fact that in studies the person administering the test to the subjects is "blind" or unaware to what is being administered just as the person being studied is unaware. In other words, double blind means that both the person administering the test, and the subject, is unaware of if they are in the control group or not.