Important note: you can not become therian. You are born as one.
This varies by person, but some signs I had were:
-Species dysphoria
Feeling that you're an animal trapped in a human body. I used to think that the human body is weak and I envied the speed, reflexes, and strength of felines. Sometimes, I wished I was some kind of feline, so I would look up "how to turn into an animal", of course, I didn't succeed because physical shifts are impossible.
-animalistic urges
Having an urge to act like an animal. My personal experience was that I would move my fingers in a similar fashion to flexing my claws. What made me realize I was Therian was when I woke up in the middle of the night and felt like running on all fours.
-Having memories of a past life
This is very rare, because some therians might not have past lives, or even if they have past lives, they might not be able to remember them. I had this experience only after I found out I was a therian, and it's only when I'm trying really hard to actively search for them.
-phantom limbs
Feeling extra body parts like a tail, non human ears, muzzle, paws, etc.
