Hi! Do you feel like you may have a close connection to a specific creature or animal? I would start out with the general gut feeling of "well, I have an interest towards _______"
If you can, think back on childhood memories or even dreams you've had. Is there a re-occurring interest or connection to any animal / creature?
For example: When I was a kid I was always very interested in sea life and water! I pretended to be a mermaid constantly, and I always felt free in the water. It's really just an internal feeling you can't shake off. In what ways do you not feel human? Feel free to PM me!
First, I'd recommend reading the other Q&A threads. This question seems to come up a lot and we tend to give the same advice.
Second, we'll need more information if we're going to be of any help. What kinds of things make you question being nonhuman? What characteristics keep coming back? Work from the inside out, not the outside in, in other words, don't have an end goal in mind. Just look at the evidence and go from there in your quest to narrow it down.