Exhaust Valve seats

[jdodd68]

I have had my truck for 5 months, and I really haven’t put many miles on it. It’s a 68 C10 with a 350 from a 70 C10. Well the other day I had to replace the valve cover gasket because of leaks. While I had off the covers, I looked at the casting numbers (3947041). I found they are good heads but didn't come with harden valve seats. The thing is, I don't know if the previous owner changed the exhaust valves for unleaded gas. I think I know the answers, but is there a way to tell what valve seat they have without taking the heads off? Since I am sure there is no way of knowing without taking them off.

How much damage will I be doing if I keep running unleaded gas with harden seats?

If valve seats have been changed (and me still not knowing) will putting an additive in hurt it?

With all that said: What would you guys do if you didn’t know what valves seats you had? (Not including rebuilding)