There is no universal way.
There are about four thousand species of snakes in the world, and approximately a quarter of them are venomous. However, distinguishing a venomous snake from a non-venomous one can often only be done by a specialist, as some harmless snakes look very similar to deadly venomous ones, which helps them protect themselves from predators.
Nevertheless, in some regions, it is quite simple to determine the danger of a snake based on external signs. For example, in Europe, there are several species of venomous snakes from the viper family. Their distinguishing features are vertical pupils and a triangular head shape that is wider than the snake's body. These traits are characteristic of all members of this extensive family of snakes.
Venomous snakes from the aspid family, found in tropical and subtropical regions, have round pupils (except for some deadly venomous snakes found in Australia) and a slender build. This appearance brings them closer to members of the blind snake family, so caution should be exercised around any snake in areas inhabited by aspid snakes.
