This is one way
rclone lsf remote:path-to-file
You'll need to check you actually get the filename in the listing though.
Doing this via the rc would be most efficient though as there is some overhead starting the backend. Eg with Remote Control / API
rclone has the perfect internal api for this fs.NewObject.. Maybe the thing should be to add a operations/newobject to the rc. This would then do the absolute minimum needed and return an error or something about the object.
There is rclone check which you can pass --files-from to.
I'm a fan of using rclone lsf on the source and dest and then using comm to find missing things.