They generally only look on Weibo & other Chinese-exclusive social media, and they do it all the time, not just while you're in customs. For something on Twitter, Reddit, Facebook etc it would need to be something really, really egregious (and you would know about it) like organising or raising funds for the East Turkestan Islamic Movement, or something similar. It wouldn't be enough to just express a political opinion that the US State Department has expressed, like "China is committing a genocide of the Uyghurs". Maybe if you're saying it in Chinese it's more likely that would lead to issues because that's what they're used to dealing with, but I think it's unlikely. They care about what their nationals are doing, and they care about what's happening on their own social media networks; that's mostly it.

My general advice for people travelling to China is to not talk about politics on Chinese social media, or if you do just talk about the domestic politics of your home country & keep in mind that Chinese people might disagree with you. That's also my advice for people travelling to any country, but it's more important in China.

All that said, if you must discuss politics on Chinese social media while you're there, the thing the censors really have an issue with is calls for action, explicit or implied. More than one very pro-PRC heritage speaker who went to China has had their Weibo posts raging against America or Japan censored because they thought the criteria were "Posts have to be pro-China", when really the criteria is "Posts can't be a call to collective action that wasn't started by the party". What the party is actually concerned about is just stopping any sort of organised mass movement that they didn't start. The CCP's point of view is that mass movements are inherently unpredictable & could lead to civil disorder (even if they're nominally "pro-China"), so they're too risky a tool to let anyone other than the state use - important context to that is that Chinese culture, similar to some other East Asian cultures, puts way more value than we do on civil order, harmony etc.

Also if your posts do get censored, it's not as big an issue as it would be here. Where I live, the government deleting my social media posts would feel approximately as serious as armed police rappelling through my windows, and if the former happened I'd at least think about the possibility of the latter happening shortly afterwards. Think something like the Christchurch shooting live feed. It's not like that in China; it's completely normal, for example, that you get angry & post something that gets deleted by a censor, & that is literally the last you ever hear of it, a lot like tweeting something against ToS. If you continue posting about it or try to get around the censorship, eventually a police officer will visit you and talk to you over tea about why you have to stop doing that, and if you keep going that's when the actual legal consequences like deportations or arrest start.