Answer:
Ok, in this case, but researchers think that their drug is better than the other one, so:
What would prove that researcher A is correct about it?
Ok, researcher A and researcher B need to test their drug to see the effectiveness of it (and to see if there are some problems with the drug)
Then, if the drug A has a bigger percentage of effectiveness or/and less secondary effects than drug B, we can conclude that drug A is the most effective and that researcher A's claim is correct.