How do people know HCN smells like almonds?
The odour threshold for hydrogen cyanide $(\ce{HCN})$ is in fact quite a bit lower than the lethal toxicity threshold. Data for $\ce{HCN}$ can be found in many places, but here and here are a couple of good references. That subset of the human population that can detect bitter almonds do so at a threshold of $0.58$ to $\pu{5 ppm}$. The lethal exposure dose is upwards of $\pu{135 ppm}$. That's a whole $\pu{100 ppm}$ range in which to detect and report the fragrant properties.