In-camera, you'll most likely have to work by your own eye. You can practice by placing a large paper or poster board with a rectangle drawn on it, in the aspect ratio you want (3:4, 16:9, etc.). Then, look through the viewfinder, fill the frame with your rectangle, and note where the cutoff lines are. With ptactics, you'll be able to visualize the working area.
Once you get back into Lightroom, you can make a crop preset (if one doesn't already exist) and apply that to your images to create the final image in the size needed.
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Cameras with live view mode capability have menu selections to select different shooting aspect ratios. The actual cropped image can be displayed on the camera's LCD screen. Check your camera manual to see if that is available. Here's an example: Custom Live View Settings on a Canon EOS 60D - For Dummies
You can also display different crop aspect ratios inside LR. This will help you to "visualize" how images will be cropped when using the camera's viewfinder. With 16x9 aspect ratio the top and bottom of the image will be cropped out. With 4x3 aspect ratio the left and right sides of the image will be cropped out.
1) In the Develop module select a normal camera image file and then click on the Crop Overlay tool.
2) Next select menu Tools> Crop Guide Overlay> Choose Aspect Ratio and select the video aspect ratio(s) you will be using.
Here's an example showing both 4x3 and 16x9 crops selected with a standard 3x2 aspect ratio DSLR image file.