How do I know if my begonia plant is poisonous to animals?

Yes it can be. Do your pets chew on your plants?

http://www.417petsitting.com/news/7-plants-that-can-harm-your-pets

https://www.aspca.org/pet-care/animal-poison-control/toxic-and-non-toxic-plants/begonia

I have a bunch of pets too with some potentially harmful plants, but none of mine care or chew on them so I don't worry.

We don't take a chance. If it can hurt a child or an animal we don't have it here. We were given a beautiful plant (Daytura) that is used in American Indian ceremonies. The bloom only lasted about a day but was one of the most beautiful things I have ever seen. But when leaning how deadly it can be we pulled it up (wearing gloves), put it in a bad and straight into the trash can. Some things just aren't worth it.

I agree with Naomie, as long as your fur babies don't chew on your other house plants they should be okay. Watch it for a few days, if left undisturbed, great, if it looks like interest is being shown towards the angel wing begonia plant, turn it into a hanging plant that will be out of their reach. We have plants all over the house including a room full of plants overwintering (five huge hibiscus and many others) and they haven't been touched.

No the cat don't touch it , its when some fall off i was hoping he wouldn't touch it.