Isso parece fazer o truque:
#!/bin/bash
mountpoints=( $(awk '$1 !~ /^#/ && $2 ~ /^[/]/ {print $2}' /etc/fstab) )
for mount in ${mountpoints[@]}; do
if ! findmnt "$mount" &> /dev/null; then
echo "$mount is declared in fstab but not mounted"
fi
done