Mechanic said the heads were replaced. Compression checks the heads right? I've been wanting to get around to getting a compression tester.
Compression tells alot of things. It tells if the valves are sealing properly, if the rings on the pistons are still good.
But valve guides are just that they are guides, inside the head that the valves "ride" in. They wear out over time and if they do then the oil can get past them and into the combustion chambers.
Common problem on chevy V8s, very typical to see some smoke on startup with those because the wear is just enough that when the motor sits some oil drips down into the combustion chamber then waits for you to start the car and makes a little smoke.
Not a guarantee thats your problem, but overall if its not smoking constantly I would say you are ok for now. Keep an eye on it.