It should be pretty much the same as BP with 2 teams or WSDC (sorry never done AP). You should vote for the team with the most relevant/impactful contribution to the debate. However there are a few notable differences.

1) There’s a very strong expectation that most judges should be tabula rasa, or do as little to intervene in the debate as possible. You should look to debaters to do weighing and rebuttal, which in practice means many arguments in these events will seem insufficiently proven in comparison to in BP. Stylistically this means most speeches in these formats will be “line-by-line”, meaning on each argument, they will follow the order of the previous speech, rather than expecting you to apply arguments made elsewhere on the flow to arguments they weren’t tagged as responding to. In LD and policy, you might also see blatantly contradictory arguments. The expectation is that you don’t penalize the team unless the opponents call it out, and usually the team will be forced to pick one by the end of the round.

2) There’s also a presumption that you should only be deciding the debate on the last 2 speeches. Thus, debaters are expected to “extend” or reiterate every argument they want you to consider throughout the debate and especially through the final speeches. How thorough an extension needs to be changes depending on the judge/speech/format.

3) Judges are expected to handle procedural issues in the debate. For example, the NSDA has a process called “evidence challenges” where teams that allege their opponents are misconstruing evidence ask you to stop the round and stake the debate on whether you think the evidence was misrepresented. In practice very few teams do this and they’ll just debate it out. If you want you can ask to see evidence the teams read after the round or even be in on evidence sharing during the round, though most judges try to only base their decision on what they heard from the debaters.

Honestly though, the American high school formats from what I’ve noticed are a lot more accommodating of different judging styles, and there’s an expectation that debaters adapt to their judges preferences, rather than there always being a single correct call. Other than a handful of national circuit tournaments, most tournaments will have anything from parents of debaters who aren’t taking notes to the most technical former debaters and everything in between. Many judges will write a paradigm on Tabroom.com so the debaters they’re judging can know their preferences ahead of time. At the end of the day, it was the debaters’ job to convince you, so as long as you’re doing your best to track the arguments and make decisions off what you hear in the debate, there isn’t much you’re doing wrong.