How do you know a spider is in danger?

Yes. The correct term is venomous, in that a spider carries venom to subdue its prey. It turns out that all true Spiders have venom producing, storing and delivery systems. But only a few spiders pose a real hazard to people. Spider bites put people at risk based mainly on the species of spider involved. The size of the spider is a bit of a factor in that larger spiders (of the same species) can deliver more venom than a smaller spider like it. There is also the factor of the age of the person, with younger people - particularly children - being more vulnerable in many cases. Certainly older people or those with health issues or some Allergies may also be at more of a risk than a "healthy adult" might be. Most spiders don't present real risks to people, but the wise person would learn about the types of spider on his "home turf" that could be dangerous. And he would be clear about how that spider looks and where it likes to live so he could take precautions and live and work confidently. Certainly he would be able to assist any family members or friends by sharing his knowledge, and would "police" his home and work areas (or work with assigned staff at a job site) to insure that it is clear of the dangerous arachnids. Wikipedia has a great introduction to the spider posted at its site, and a link is provided.