TIL that you can see quite a bit of useful information in /proc/meminfo:
cat /proc/meminfo
MemTotal: 16251680 kB
MemFree: 173564 kB
MemAvailable: 10696060 kB
Buffers: 3732 kB
Cached: 10987260 kB
SwapCached: 4120 kB
Active: 6651376 kB
Inactive: 8841488 kB
Active(anon): 2257272 kB
You can also see the outstanding writes that haven't been flushed to disk:
cat /proc/meminfo | grep -e Dirty -e Writeback
Dirty: 256 kB
Writeback: 0 kB
This means that there is 256kb that is waiting to be flushed to the disk and 0kb currently being written.
This is helpful when trying to move things to a usb and you have no idea how far the process is in actually writing things to the disk.