Look into his eyes! If they are round, then he is safe, and he is there eating slugs and other bugs that would eat your garden.
I couldn't tell much about the head shape from the photo, nor the eyes.
But, if not poisonous, and no threat to livestock, I would be glad to have it there. Also look at his tail, non-poisonous snakes have long tails, poisonous ones are either stubby or have rattles on them.