What are the parameter naming conventions for packages?

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How do you determine the correct parameter names to send to a node?

I've noticed some variability in naming conventions around package parameters. Specifically, I'm trying to use stereo_image_proc and image_view, and each seems to use a mix of parameter names with and without a leading underscore. The wikis fail to mention which parameters require a leading underscore, and running package with either produces no explicit error, so it's difficult to determine which to use. However, occasionally a node will complain with a warning like:

Try restarting stereo_view with parameter _approximate_sync:=True

indicating an underscore should be used. Although both wiki pages mention the "approximate_sync" parameter, neither mention it must be prefixed with an underscore. How do you determine the correct usage?