how to find out the latest hardware version available for a VM

Hi, I'm having an issue upgrading the hardware version of virtual machines.

my $vm = Vim::find_entity_view (view_type => 'VirtualMachine', filter => {'name' => $vm_name});

$vm->UpgradeVM();

Invoking the method UpgradeVM I sometimes get an error saying: Virtual machine compatibility is already up-to-date

I can get the current hardware version checking $vm->config->version, but I don't know if it is the latest available hardware version or not. How can I find out which is the latest hardware version available for a virtual machine before upgrading it?

Thanks in advance.