I realize the topic of this entry isn't quite up to the tone I've set for my blog so far. Oh well. It was going to happen eventually. Now, moving on.
1. The bottle blonde's hair is of a perfectly even, uniform shade of blonde. Very few people have hair of a perfectly uniform color - blondes less than other people, because the color is so sensitive to sun. Dye, on the other hand, tends to flatten the range of colors. This only really works when the bottle blonde dyes her own hair, or has it dyed cheaply. An expert colorist will try to mimic the color variation of naturally blonde hair.
2. The bottle blonde's hair is too shiny. When I had highlights in my hair, this was what I noticed most - my hair was unnaturally shiny, all the time. Some people must have naturally shiny hair, and some natural blondes may gloss their hair regularly enough to give the same effect - but if you ask me, this is a nearly foolproof indicator of a bottle blonde. Expert colorists will want to enhance, not dampen, this effect for their clients - shiny hair is good, after all - so even if they manage to realistically blend color, the bottle blonde will walk out of the salon with unnaturally shiny hair.
3. The bottle blonde's hair does not change with the seasons. Natural blondes will have noticeably darker hair in the winter, and noticeably lighter hair in summer/fall. I imagine that this, too, is something that an expert colorist would compensate for - but then, it would take a pretty sophisticated client to want it.
4. Beware of assuming that dark roots always mean the bearer's hair is not naturally blonde. Sometimes hair that grows in dishwater blonde will lighten considerably, especially under the influence of sun and saltwater. Roots are only a good indicator if the line between dark and light is crisp and even along a part in the hair. Similarly, don't assume that anyone with dark eyebrows or eyelashes has dark hair - I can't be the only natural blonde on the planet who darkens my eyebrows and eyelashes every day.
5. Basic sloppy salon procedures - i.e., if highlights are too chunky, if they are scattered through the top layer of hair like silly string, if they are placed too evenly, etc., etc.
6. The people who masquerade best as natural blondes are people who are almost, but not quite natural blondes. They have the right complexion, usually, and especially if they have money to burn on a good colorist it's going to be nearly impossible to spot the difference. However, one of the above methods should work.
These tips aren't foolproof, but I venture to say they'll almost always lead you to the correct conclusion.