2)If the ESX is in Vcenter how can i check under which Vcenter it is.
Easiest way is to connect directly to the ESXi host with the vSphere Client. You will get a pop-up window that tells you the vCenter Server that is managing this host.
1)I just want to know is there any possibility to check what OS is running on my host machine(ESX).
After completing the previous step, you can list the VMs running on this host. One of the columns is for the guest OS.