If coming from the guides, you mght see oil puddling on top of the valve (the valve in question of course needs to be closed) after the engine has sat for some time.

Specifically, I had a situation on my SB factory heads where using the hard Teflon seals just didn't work right. This was on re-done '596 castings, so proper guide liners installed, new valve job, etc. Needless to say, I could tell that the oil was getting sucked in through the guides, the tell-tale result was the puff of blue smoke on startup and when sitting in traffic (think Woodward Annual Cruise) I literally could see the exhaust starting to show some blue smoke after creeping in traffic for a good 45-60 mins.

Eventually I physically confirmed my suspicion by removing the carb (after the car sat for a few days) and snaking down a USB camera through the intake runner port until I got the the valve stem. Indeed, the oil was all puddled on top of the valve, the runner itself was clean though, so I did not see the oil weeping path coming out of the intake/head junction (assuming that the leak was happenning though a miss-match of the mating surface).