Actual proof is the most important proof in Buddhism. It can be ascertained in two ways. One is by examining the history and teachings of a sect and the biographies of the people who practice them. The other is by actually practicing Buddhism and seeing whether or not the methods and theories of the teachings have actual validity in one's dailly life.
It is important to note, that the principle of causality indicates that people will not in the long run actually benefit from incorrect teachings. There will be subtle "poisons" or errors in their lives despite their innocent faith in their teaching. So while "actual proof" may be found in flawed teachings, that actual proof will be "inferior" to the actual proof found in a "superior teaching." Of course there are times when teachings may be equal, with some parts of all of them erroneous, or all of them equally profound. In that case "Actual Proof" becomes an inward and subjective matter. Making this subect even more difficult is that people may focus on flawed teachings while ostensibly practicing a "superior teaching" or may have the good sense to ignore the flawed teachings of their own "school". Actual proof is both the most important of the three proofs and the most subjective.
On this website I spend a lot of time on "literal proof" but actual proof is even more important ultimately. Ultimately lies are deadly.