|You can visit this site for a very good snake identification tool. Or see my Florida snake photos page for 80 of my best photographs.|
Or, most online searches describing the appearance and markings of the snake will yield good results. For example, you could type, "black snake white belly" or "yellow snake with stripes".
Or better yet, just take a digital photo of the snake, and email it to firstname.lastname@example.org and we will tell you exactly which snake you've caught!
Overall, venomous snakes tend to be a bit thicker, or have very broad heads, or have rattles on their tails. But there are of course exceptions. The rule of "triangular head" does not apply, as many harmless snakes have triangular heads. The best way to know is by knowledge or research. And of course, if in doubt, leave the snake alone! Most snake bites occur when people try to grab or kill snakes!