Doing exactly that is called 'concern-trolling'

It is, but it’s easy to undermine without resorting to accusations of trolling, by pointing out that attacks on the character of a source of information rather than critique of the information is unconvincing. It also helps as ryanlol did, to point out non sequiturs such as injections identity politics into the mix for no good reason.

Especially the way this is phrased. “I am a liberal” is not something liberals actually say.

For Reddit there is a Chrome plug-in that lets you highlight users who frequently post on specifically troll-heavy subreddits such as The_Donald so you can spot this kind of shit easily.

True, but assuming that everybody's general world view matches our own is what got Trump elected in the first place.