How do you know when a bobcat will attack?

As with any wild animal you should always keep your distance. Making your self look big, loud shouts, and aggressive movements will generally scare off a Bobcat. If that doesn't work then the next course of action would be to defend yourself either with a weapon or hiding if possible. If it's just a sighting then notify your local wildlife agency and have date, time, location, and possible prey sources (small dogs, domestic cats, livestock, etc.) information to give them. Bobcats are "scaredy cats" most of the time unless provoked, frightened, or near their litter.