Personally, I avoid the term spirit animal, namely for culturally appropriative reasons.
Other terms are power animal, spirit guide, fetch, familiar spirit, totem, flyga, etc. These all can mean the same thing, or completely different concepts.
That said, I'm under the opinion that a person has a totem with a archetypical spirit of a species or form, and possibly it can be more than one, and it's privy to change. It doesn't even have to be an animal. flora, land-forms, weather, etc, can all be totems as well.
For example, I have fly agaric as a totem, which is a fungus, as well as the magpie. Previously, Loon and birch were close totems.
Another thing to realize is that a power animal, spirit guide, and/or totem, is a two way street. You are in a relationship. This can be hostile, friendly, and start one way or another. You aren't there to just reap the wisdom of the animal (or in a lot of cases, satisfy your fondness for it). And you or it has to initiate contact to truly know.
Now how do you determine if something is suitable? Build a relationship with the species. Go out and find the animal/flora/landform. Go into the spirit world and meet with the totem. Or go there with the intent to find one that wishes to have a relationship with you.