How do I find out what filesystem FUSE is using?

I found the answer provided by in the comments by Don Crissti to be the best

lsblk -no name,fstype

This shows me exactly what I want and I don't have to unmount the device,

mmcblk0     
└─mmcblk0p1 exfat

See also,

• man page on lsblk

In general, it is not possible to go from a FUSE mount point to the process implementing it.

If you know something about how that filesystem works, then it might be possible. You have to track the device side, not the mount point. For example, in your case, the FUSE filesystem is exposing a filesystem on a block device, so you can look for processes that have the blockd device open: lsof /dev/sdb1 or fuser /dev/sdb1. Similarly, with SSHFS, you can use lsof or netstat to look for a process that has a connection to the right server, etc. This gives you a process ID, and ps can then tell you what program that process is running.