On a L28 without excessive blow-by, you shouldn't hear anything.
If you have blow-by you should hear a growling noise and putting your hand plugging the hole might even cause pressure to build that you will notice when you remove your hand on a car with huge amounts of blow-by.
There are varying degrees of this and what I described is "whole engine blow-by" and just a single cylinder with the amount of smoking mentioned would manifest itself as muted pops through the oil cap hole instead of a groan.
Usually, if an engine has enough blow by to make clouds of smoke, then you should expect that it will expel exhaust gases past the rings into the crankcase.
A properly running L28 with stock breather hose routing may actually misfire as removing the oil cap will allow air that was not metered by the air flow meter to enter the engine and cause a miss in some cases.
Some guys run into the misfire when they remove the hose that goes from the valve cover to the induction system between the air flow meter and the throttle body and instead use a filter on the valve cover barb.
If you don't have any of the "blow-by" symptoms described, then rejoice that you instead have leaking valve stem seals.
Of course, removing the head to fix the seals will also allow you to inspect the bores which might show some scuffing.