/etc/fstab is world-readable, so you can just
cat /etc/fstab and see if it's there.
The partitions may be listed in
/etc/fstab by LABEL, UUID, PARTLABEL, or PARTUUID instead of by the device path. In this case, you might not be sure if any of the entries correspond to
/dev/sdx or not. Running
blkid /dev/sdx should show you this information about the partition, if you have permission. (Non-root users may need to be in the
disk group, or whatever group owns
/dev/sdx.) If you don't have such permission, you can still see what device e.g.
UUID=aa1a69af-0bad-48b8-a659-95f1daceaceb corresponds to by
ls -l /dev/disk/by-uuid.