How do I find out server model and serial number from vmware ESX

You can usually get the model from the VM client (3.5 Infrastructure client, 4.0 Vsphere Client).  It will be listed under the "Summary Tab" in the General section.

I don't think you can get the serial number.  

You can connect to the ILO port of your server to gather information.  Som basic, but he serial number is also listed there.

you can also install HP Insight agent into the service console and connect via browser or even easier take a look at hardware status tab it shoudl be there