Has the cylinder head been removed yet? If so, you're probably seeing a little coolant that dribbled down the oil return holes as the head was lifted off. Any machine shop worthy of spending money with will have a way of pressure testing the cooling passages. I used to make a couple of block-off plates for small block V8's with the pistons removed and the heads installed and plumb a garden hose into the cooling system if I suspected cracks. City water pressure in a block or a head finds leaky spots in a hurry!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!