As I get older, I tend to rely less and less on this method as I've found I need to get closer & closer to see the snake's (assumed) intent. I wind up well within striking distance before I can look 'em in the eyes.
Except for coral snakes, which are not pit vipers and live all over Florida and much of the American South. Coral snakes also need a different type of antivenom than the generic CroFab that works for American pit vipers. Here are some visual ways to distinguish a pit viper from a non-venomous snake.
Poisonous or not I'm always on the lookout for snakes when walking outdoors. I'd hate to step on one and get bit, by even a nonvenomous one. I just realized I am 50 years snake free. I used to catch and handle them and kept a couple as pets, until I had an incident.