One of the most difficult things I found about photography when I first started was being able to picture how a scene would look when I used a particular lens.
I would have an idea of what I wanted to capture but not know which lens I needed to bring my mind's image to life. So I would stick a lens on, zoom in and out, swap it for another lens and zoom in and out again until I got what I was looking for.
As I got used to the effect of different focal lengths I found that I got to the stage where I could instinctively reach for the lens I wanted to create the effect I could see in my mind.
One of the best ways I found to help me pre-visualise the effect of a lens was to put the lens on the camera and actually look through the viewfinder. Sounds pretty obvious doesn't it but it didn't seem so at the time. I found it worked the best with a telephoto lens. Rather than squinting at the horizon trying to see if there was anything to photograph I would stick a long lens on and scan the distance looking for interesting subjects.
The image above is one such example. It was taken in the tiny pilgrimage town of Pushkar in the state of Rajasthan in India. I was having dinner in a cafe and looked out to see this lovely light. I didn't want to let this light go to waste but couldn't see anything to photograph. So I put the 300mm lens on and started looking around. My eyes settled on the domes of this lovely building spotlit by the afternoon sun and the stunning mountains in the background. The bird flying past was a lucky fluke. :)
Try it yourself. Get yourself into a position where you've got a good view all around you. Tops of mountains or buildings are great choices. Put your longest telephoto lens on and just scan around. Hopefully you won't get arrested for being a pervert and spying on the person sunbaking naked in their back yard, and you'll come away with some interesting compositions and a new understanding of how your telephoto lens sees the world.