"It's the way to educate your eyes. Stare. Pry, listen, eavesdrop. Die knowing something. You are not here long." - Walker Evans
If you have a smartphone, there are utilities for this. I use "viewfinder"for android. It allows me to define focal length and film format used, and it shows the "frame" in the camera image of your phone; you can even have multiple frames displayed together, e.g. all the frames of the lenses you have with you at that moment.
I use a Linhof Multifocus viewfinder to determine focal length and I use L croppers for composing. I usually carry about 7-8 lenses so the linhof multifocus comes in handy.
If switching between camera systems (i.e., 120, 5X7, 8X10), not photographing everyday, some kind of shortcut using any particular system may speed composition. However, too often I see people wandering around with these framing deals, attempting to "find" a composition, not "seeing", or having any real feeling for their stalked subject. These devices are used as a crutch, for my previous simply stated reasons. As previously posted, restricting one's choices to one or two lenses at most – and taking the mind off too many equipment choices in the bargain – will likely do wonders for your composing skills, not to mention your ability to choose a lens. Not everyone is equipped physically, mentally, emotionally, or aesthetically to produce worthy photographs of every type of subject they come across. Likely as not, you may find the particular style of photography you are best at.
My method ... Hmm, this lens might do it ... look thru groundglass ... er, think I'd like to try something else ... Hmm ... life was so much better when I only owned one lens ...
Shoot sunrises/sunsets and get to your location late.
I guarantee that you'll learn to understand your vision versus which lens very quickly! You'll also learn how to set up, focus, work out exposure and shoot quickly as well...
You miss 100% of the shots you never take. -- Wayne Gretzky
I downloaded the ViewFinder for android app, but I cannot get it to work. How do you input data? My screen is showing mo popups or indication of how to use it. I did not get asked for money when I downloaded it, maybe that is the problem somehow.
Smart Phones... iPhones... ehhh!!!!!!!!!!
I'd rather buy a cheap DSLR and cheap lenses that span all the equivalent angles of view that my film cameras/lenses offer. Phoney-babies can't offer that much versatility. Not yet anyway.
The above stated... Thirty years ago I'd have said I don't need no steenkeen visual aid!! But, after these many years and significant sight loss, I might. We'll see soon enough.
Look around, look behind. Maybe sit somewhere in the scene and let your creative brain catch up with your walking or driving brain. After a period of time when the location is starting to sink in and you can actually see what is in front of you examine the scene for all the components that make the image that struck you in the first place. For some the process of slowing down to catch up with the scene doesn't take long at all but others can spend hours in an area that at first impression held nothing before the landscape begins to reveal its component parts. I carry a viewing frame and find it very useful. When I reach that point I'm starting to see the scene in front of me I start contemplating the refinement of lens choices. My viewing frame has a 5x4 opening then a knotted string at the focal length of each lens I have. After all this is large format so if this seems a bit slow you can always get a video camera.