There is not any automatic bandwidth detection in rclone.
Sounds like you want to do more quality of service in your router or networking device?
There isn't any real way a client device would know available bandwidth as there are way too many factors to figure that out and figure out what changes continuously.
What problem are you trying to solve? That's probably a better question than trying to start at a solution.
I have a Proxmox server hosting several containers and vm's. If one of them is uploading a large file to the cloud, then my network is completely taken by that upload. I am using a webdav connection to the Stack cloud of TransIp.
The DavFs2 driver eats all bandwidth it can find. So I want to use Rclone instead, because it has a bwlimit option.
Does this clarify my question a bit?
In the meantime I discovered that rclone handles the traffic way better than the webdav davfs2 driver. The network is not completely blocked when rclone is active.
So for now I am a happy user of rclone. bye bye davfs2!