The tuber type, when you go to anyplace that sells bulbs in
spring, look there. They will have begonia tubers for sale, so you know what they look like. If you dig it up, it will have a nice tuber (like a small brown potatoe) on the bottom of the green
plant. If it is a fiberous begonia, then the roots are fiber roots, like hair roots. I see that you live in Ca. and that is why the
plant survived. Here they would freeze, and not come back, tuber or fiber type. Here we take cuttings of the fiber type, and the tubers, we dig & store in a cool place for winter.
Darlene