If regular sugar (at least in the US) is not specifically marked "cane sugar" then it comes from sugar beets. Some people maintain that cane sugar is superior, others don't care. I can't really tell a difference, but I always buy cane sugar. Gues I subconsiously believe the hype.
Yep, if the bag-o-sugar doesn't say "cane sugar" on it, it is beet sugar. My thinking is that if I'm buying butter (not margarine), name brand flour, name brand baking soda, real (not imitation) vanilla, and real choc chips to bake my cookies, I'm FOR SURE not going to buy beet sugar.
Pick up a bag in the store next time, twirl it around to the ingredients, and take a looksie. If it's the generic or "cheap" brand, it's probably beet sugar.
Not bragging at all, but my daughter did win first place at the state fair with our choc chip cookie recipe, using the real stuff.