Look at his head...it's straight with his body, no definition. Venomous snakes have a much different shape to their head...I always think of it as heart shaped, but whatever, lol. And you can tell by their eyes, if you want to get that close to them.
As previously mentioned 'head shape' (triangular) is a poor indicator (by itself) as to whether or not a snake is venomous. There are many harmless snakes that either have a fairly triangular head or will flatten their heads to make it look so. The elliptical pupil of the eyes is another clue (for the pit vipers), but again...anomalies exist so that (in itself) should not be used to positively identify a snake.
If we are to use a 'single' feature as a diagnostic tool..it should be the presence of a 'pit' between the eye and nostrils. All of the indigenous Pit Vipers in the U.S. have this. (Rattlesnakes, Copperheads & Moccasins/Cottonmouth).
The Coral Snake (not a Pit Viper but also venomous) does NOT have a pit, has round pupils and does not sport a triangular head....but fortunately is colored such that it is readily identifiable (if you know the pattern).
Lastly (the indigenous snakes of the U.S.) if venomous will have a single row of scales after the Anal Plate/Vent, harmless snakes have two rows.
Of course, many of these identifying features require you to be quite close to the snake in order to make a conclusive identification. So making an I.D. under
field conditions becomes understandably difficult for the layperson.