One easy way to identify the virtual disks (.vmdk) associated with an virtual machine is opening the virtual machine settings, on Hardware tab, click on Hard Disk # and on the right panel, see the datastore, folder and name of virtual disk.
You will see something like this:
Another way is using RVTools, just connect to the ESXi host and look for Zombie vmdks: RVTools - Home
After locate the possible "not used" vmdk, if you still unsure if it is utilized, you can rename them before delete.