how to tell if heads have harden valve seats

If your not going to put harden seat in them mix so leaded racing fuel into it, for the first While. Once you have the lead coating on the valve, they should be good for a long while.

When a valve job is done, that lead coating is removed.
You need to get that coating back on.

like a cam and zinc additive. The more "spring pressure", the more a harden seat or leaded racing fuel is required!