A shift is any temporary change when you feel more like your kin and less human. There are many ways this can be. If you temporarily think/feel more like your kintype, that's a mental shift. If you dream that you are your kintype, that's a dream shift. Etc. You can find a lot more if you just google this.

Not all otherkin or therians shift! Some are constantly on the same balance of human and other, without any noticeable change. That's fine too.

And how to tell whether you "just pretend" or not.. Well, the same way you tell with other experiences, moods and feelings. Can you tell the difference between actually being sad and only pretending to be sad? Actually experiencing pain or only pretending? Same skills of evaluation and introspection apply here.

I'm a wolf therian. I experience mental shifts now and then. Some are mild, some are intense. During mild shifts, I can still talk and walk on my hind legs and understand that I'm human. But my instincts feel stronger, and I have some urges to do typical wolf stuff (sniff around, look for prey etc). During intense mental shifts, talking becomes difficult, I have trouble recognising myself in the mirror, I move like a wolf would, I'm not interested in human activities at all, and I avoid unfamiliar humans.

But mental shifts vary a lot between different persons, depending on kintype, context, integration between human side and kintype, etc.

But instead of thinking of mental shift like a "this is an achievement to reach" (it isn't), think of it like a word to describe something that happens by chance to some people. Kind of like mood swings or stimming aren't things you get medals for, just a fact of life if your brain works in a certain way.