If there is a distant streetlight, then use live view to make the point of light as small as possible!
The important thing is to use manual focus so that it doesn't change when you recompose, or take multiple shots. You only need to refocus when you zoom.
If there are no distant streelights, then...
With wider focal lengths (up to about 70mm), you only need to send a friend into the landscape 100m or more and focus on the torch in live view. Everything will be sharp up to the horizon.
Another way is to use a laser pointer, find the dot in live view and tweak the focus ring to make the dot as small as possible. This works extremely well and just for a laugh I demonstrated I could check focus nearly a mile away. The green lasers used for star pointing are the best as you can where the beam is heading.