Rebuild cylinder heads

I bought a 340 motor with paperwork showing that the block had been professionally rebuilt, but later learned that the heads were rebuilt by the seller himself. I wasn't able to hear engine run before I bought, so I took a chance since it was a good price.

I've since installed the engine and have put a few thousand miles on it. Engine runs and sounds strong, but I'm burning oil. I'm guessing that the problem is either valve seals or valve guides since the heads were rebuilt by seller. What symptoms should I look for to determine the source?