LandToy80-
As far as being able to tell if one's oil filter is plugged, I imagine you would note a pressure drop equalivalent to the initial opening setting of the filter's bypass valve. This would most noteably be when the oil has warmed up and thus better able to bleed out through all the bearing clearences. I say this in theory.
Another thing that can cause pressure drop however is the shearing of the oil, which can thin it out of grade. A UAO would probably help determin both with its viscosity test and number of insoluables. Either way, as a precaution I'd follow along the lines of manufacturer recommendations and what other's experiments have proved - but take it with a grain of salt as not all operating conditions state of repair are the same.
There's been a bit of experimentation to see flow restrictions caused by oil filters here, and there has been much discussion of possible bypass valve intervention. Use the search feature to see what pops up "filter testing", etc.
And as far as filters needing replacement...it's only a matter of time before things plug-up, or worse if you've seen some of the filter pics that show medium deterioration, which I hope is a very very small percentage. It's cheap insurance not to push things too far - "but than again what is that condition exactly," I suppose you ponder.
