Well, in the case of the link above, via consequence. In law, generally you can identify winners and losers.
When you position it on politics, such as the presidential debates, it goes again, by who wins the election. If you look at my posts on the other site, I was judging Hillary and Trump on the basis of who was likely to win over swing states, and energize the base in areas where they really needed to get out the vote.
Political analysts all decided Hillary won on academic points, I pointed out Trump bare minimum tied first debate, easily smashed the second and third. There is no point in arguing it otherwise at this point, Trump clearly won, but up until election night, the "general consensus" was he lost. Just, that was obviously not the consensus of the public.
I like to test my skill set in reading international policy and war, so will often make statements that takes a year or two to take effect, and isn't obvious in advance. One example, later on into Trump's term, he is gonna give in to everyone's surprise and agree to a UN peacekeeping mission. You can obviously determine the validity of this down the road.
Volitaire made a joke about a doctor in Zadig, think it was Hermes, who diagnosed that the main character was destined to loose his sight (or die, been a few years since I read it), but the patient pulled through, came out in a clear bill of health, and Hermes became crossed, rejected his good health, and wrote a book explaining why he should of lost his eye sight, but Zadig refused to read it. Many of the types of debates people engadge in can be figured out suchly. If someone refuses to admit to it, everyone knows the truth.
Now, if it is on something not checked by facts, and sits in the realm of mere opinions, it depends on what one is getting out of it. You can have everyone take home the gold, or much more probably, everyone looses, cause everyone is a looser.
In face to face table discussions, generally people have positions but also want to be convinced otherwise if wrong. Not always, but generally. Usually after two hours, a dominate position starts to emerge, through the collapse of alternative arguments.
On a forum like this, one can infinitely reject normal discourse. I did it just yesterday, when I was having a discussion with a nietzschean who decided ethics didn't exist, because it was a invention of the elites, to control the people, making them like herd animals. Now, if you know me, not a big fan of the nietzscheans, but on very rare occasion, I'll come across a good one. Never happened once on Ilovephilosophy, but it happens. I studied a more classic ethics, based on warfare first, lots of actions quickly kill off a state, some don't. I obviously base it off of that, so when I see some confused retarded nihilist start claiming there is no such thing as ethics, then
**** about mean government and emminient collapse of society, I obviously look at them like they are a little retarded. Joker did this a lot, but went easy on him, but I'm increasingly less persuaded, so I just started stating everything he noted from that point on was the invention of the elite to control the masses, making them into a herd. He got very tired after a while. Goal was to get rid of him, as I know from commentaries on Sextus Empericus, that these kinds of thinkers go absolutely nowhere, but I could of explained how he was abusing syllogisms (in fairness, I started to) to write off any argument, while defending his own without merit. I could if gone after him on many other fronts, such as saying logical fallacies, or demanding him to point out such a occasion in the historical record, and provide proof it is universal, but preferred the denial of service approach, if only to let it dig into his head how
**** retarded that formulation is. Such a debate isaimed at catharsis down the road, is a technique aimed at forcing someone to consider their logic and motivation much farther down the road. Think of the meeting with Buddha in Herman Jesse's Siddhartha- it wasn't aimed at winning him over then, but decades later. If you and Trixie sat in judgment at that time, either of you would say buddha lost. Did he? No, he left a idea that ate at him slowly over time. Dialectics is larger than the debate.
None the less, if you see good analysis if debates, I encourage you to post them here, so others can learn from it.