The guide you linked provides the following commands to tell whether your disk is HDD or SSD:

cat /sys/block/sda/queue/rotational
smartctl -a /dev/sda

However, Linodes are KVM guests, so the output of these commands are representing the emulated (virtual) disk, not the actual characteristics of physical disks on the host.

After the beta program ended, all Linodes have been running on SSDs since the official announcement in April 2014.

Test your drive speed:

dd if=/dev/zero of=/tmp/test bs=64k count=16k conv=fdatasync

16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.987893 s, 1.1 GB/s

Looks like NVME to me.