Instead of "how do you know I'm not as smart as they are" lets turn it around. What makes you think you are as smart as they are and therefore should be arguing with them without more consideration for the fact that you could be wrong?
As for your intelligence compared to theirs I actually do have a very good idea, as do most other people. They are clearly more intelligent than you, and by a large amount. That isn't an insult. They are more intelligent than the vast majority of us, and by a good margin. In a case like this where the disparity is so large you should be able to see it too like most others can. The fact that you can't is scary. The fact that you can never see when anyone else is more intelligent, ever, is exceptionally scary though.
But sometimes it is hard for people (all of us) to judge the intelligence of those more intelligent than us because we don't have the intelligence to do it (another catch 22). But forget about intelligence. What about knowledge and experience? They clearly have significantly more knowledge and experience (you aren't even in the same universe) on the topic (physics) than you do (and that is according to you by your own admission). So what is your justification for not having more consideration that someone, multiple someones, with a universe more knowledge and experience on the topic you are arguing about might be the ones who are right and you might be the one who is actually wrong?
The only reason I can think of to want to argue with someone with a ton more knowledge and experience on something without giving major consideration that you could be wrong is if you think you are a whole lot more intelligent than they are But you already essentially said in your quoted post above that you don't see any way how you can tell how smart someone is, at least not from forum posts anyway, so that obviously can't be it. So if you don't think you are far smarter than they are (or correct me if you do think you are) then why is it that you aren't even willing to consider that you might be the one who is wrong on the disagreement with someone with substantially more knowledge and experience on the topic?
As you said, on occasion maybe you might not have any reason to believe the other person/s would be right over you (although this isn't even close to being one of those cases). But when you feel that way every single time, you have to know that is a problem. When you never see anyone else as more intelligent than you, ever, and you never see anyone else as more knowledgeable than you about anything, ever, and when you never see anyone else as more experienced than you on anything, ever, well it is then that you know that the problem is with you and your perspective and your grasp of reality.