Botox comes with white label with the Allergan's Botox logo and a purple top. When you examine the bottle closely there is a hologram across the label.
Because you probably don't handle the bottles all that regularly, and it would be pretty difficult to spot a fake, your best bet is to go to a reputable dermatologist. I doubt an established physician would risk his/her practice by using a fake product. In the New York area, I see Botox ads with prices that are too good to be true. Chances are, they are underselling everyone else b/c they are not using real product, purchasing product from who-knows-where, or injecting you with smaller amounts of medication than they claim. To get the real thing, pick the right doctor.