Tips to prevent the disease or stop it from progressing. One, high-five the pawn as much as possible. The longer the disease is active the shorter the window for high-fiving. Don't always run ahead and force them to pop in closer to you... if one starts lagging behind, wait for it like a quest NPC during escort. Sometimes they get "lost" (run around in circles after losing sight of you), and when you go back for them, they soon catch up. Don't kill NPC characters. Part of it is that they learn from you... you show them to kill NPCs and they will, do it often enough and you can cause the disease to start in previously uninfected pawns. If you play trickster, pawns get annoyed (unless you play the class a specific way), because they are doing all the work and generally don't like it. If you get pawns that like to sit around, they were probably influenced by a Magic Archer taking out things without them having to do anything. Pawns can get lazy as a learned trait.

"It's difficult to describe, but I feel as if I am possessed by a greater power" ... they're infected.

At first, the reddish flashing is faint. A slight color change, mostly in the pupil. Over time, it covers both the pupil and the iris. Instead of being faint, it becomes a vivid red glow strong enough to completely hide the eye color.

Sometimes, usually after sleeping (inn / campsite / house), a different pawn may start saying that. An eye check shows the pawn newly speaking of power has eyes flashing red, instead of the other's.

If transferred, it seems to stay at the same level as the previously infected pawn had.

When I see a vivid glow, I drown the infected pawn. So I haven't seen how it ends.

Also - they can get it from fighting drakes. A slower form of possession, maybe? It doesn't only happen when returning through the rift.

Maybe they always get it from dragonkin? And bring it back, when it happens elsewhere?

Neither "Power courses through me" in combat nor "I am unlucky to meet you a second time" in the rift is an indicator. THey say it even without the Plague

this alone has made me not hire pawns if i can't see their eyes

My pawn gets annoyed or sassy even without the Plague, I think you overinterpret it.
She also quarrels sometimes with other pawns.

One time she sat down and another pawn commented that she cannot rest because she has to protect the Arisen. My pawn only commented in an annoyed tone: "Yes, I know, thank you" xD

Some inclinations are just sassy per se.

The Rift will become very corrupted at some point and that's an indicator of something to come :^)

So it's actually REALLY easy to tell. You notice patterns in how the Pawns will address the Arisen, while sometimes they may be a wee bit sassy they never doubt your judgement and follow you with blind trust. An infected pawn WILL question your strategies during combat, and the BIGGEST tell is if they give you ♥♥♥♥ in the rift when you hire them (a moment I had where I hired a pawn only for her to say she was BURDENED with me twice and immediately getting the Dragonsplague pop-up). For as sophisticated as the AI can be the dialogue options are still way too easy to read in my opinion and hopefully they add a little more to make it trickier to spot in the future.

I created my pawn as a blind archer with both eyes closed (a double Guts I call it with a cut an inch higher) in the CC before I knew anything about this. It might affect how often people choose her in the rift, but I kinda don't want to change it.

You CAN see if a pawn is infected while in the rift.
The only sign you need is to look at their eyes. In the rift their eyes will still pulse.
Otherwise, before you sleep in town, just line your pawns up for inspection. Just check if their eyes are pulsing red.
Also note that your pawn can get plague simply by entering a riftstone. So check them after you leave to be safe.

Otherwise, its really not as big a concern as people are making it out to be. If a pawn goes on a muder-spree you just sleep in your bed for 7-14 days and everyone comes back to life.

Post script:

There is an inconsistency with the animated effect on clothing. Might not be related to the plague.