Customers have two primary methods of managing TLS certificates for their ESXi hosts, they can either use the built-in VMware Certificate Authority (VMCA) which is part of vCenter Server or Custom CA Certificates. I will not go into the gory details, but you can read more about the options here in our documentation.
A question that I had received recently was whether you can determine the type of certificate an ESXi host was provisioned with and whether this could be programmatically retrieved using the vSphere API? The answer is yes. In vSphere 6.0, we introduced a CertificateInfo property which contains a number of fields including status, issuer, expiry and subject details and by inspecting either the issuer or subject property, you can determine the type of certificate on the ESXi host.
Here is a screenshot of the data using the vSphere MOB for an ESXi host that has VMCA-based certificate:
Here is a screenshot of the data using the vSphere MOB for an ESXi host that has custom CA certificate: