You could easily see it in how the rules were applied to posts or comments in big subs. Personal attacks against conservative people were allowed to fester for days, while those against leftists were removed immediately. I got banned for reporting stuff and asking why it wasn't taken down days later. That was 2014.

You could see it when people were critical of Obama. Or how articles were buried purposefully and if you tried to resubmit they would get removed for already covered story that was locked at like 12 karma.

There was a post a while ago that showed the biggest mods and power users controlled something like 80% of the content on reddit.