Ah, the joy of transitive/intransitive pairs. Hopefully this post is less ramble-y than it felt to write.
(I wrote most of this before I saw @SleepyOne’s way more concise comment so screw it I’m posting it anyway)
So the article posted above mentions and dismisses patterns for remembering transitive/intransitive pairs, but it only actually “debunks” half of them? Which are not the ones I’m about to share and have served me well… So anyway, one of the things that helps best is not just a single mnemonic, but recognizing a general pattern.
If you’re aware that ある is (on of) the Japanese verb for “to be” or “to exist”, and you’re vaguely aware of the concept of verb transitivity, then this becomes easy.
上がる is to rise. The thing is rising. It is not raising something else up. The subject itself is rising. It is both the subject and the object. The action is self-contained. It’s intransitive. And you know what the simplest intransitive verb of all is? ある. And that rhymes with あがる. And no that’s not a coincidence.
From there you can infer that 上げる must be the transitive version, so that would be to raise (which you do to something else).
Similarly, if you have a pair of verbs and one ends in す, it’s transitive. You can remember this because that す ostensibly comes from する which is “to do.” Which is about the simplest way to describe affecting another object. You “do” something to it.
Granted if you see an える verb in isolation, there’s not too much of a general rule for whether it’s transitive/intransitive, especially when inflections come into play (though that’s not usually an issue on WaniKani). So せる and める verbs might “always” be transitive, but that’s only if they’re part of a root pair, instead of being the potential forms of some intransitive verb.