they tend to have a single main stem, while fibrous begonias used in bedding develop multiple stems from the base. Tuberous have large leaves and large double flowers. Fibrous begonias have many small single flowers. The most common tunerous begonia sold is Non-Stop. Getting tubers the first year can be difficult.

I also have come across more elatior begonias this year. These are year round plants. Here is a good site for basic plant ID:

Here is a link that might be useful: Begonia ID

Pretty good overview, bmchan. Is it yours? I would quibble with one thing, tho'. If the temperature ranges given are accurate I'd better go out and ditch all my (many) begonias. Temps 50 to 72? That's a day in January here! I feel extravagant when I turn the AC to 79, and outside it's pretty consistently in the low 90s these days, with lows in the middle 60s. Humidity so high I grow things that need terrariums indoors, just outside without any protection. Have a tiny little Buttercup in a 3" pot that's happily blooming. Can't grow tuberous here, but most everything else grows like weeds--as long as I get out there with the hose regularly!