How Do I Determine if Coolant is in Engine Oil?

I think coolant is contaminating my van's engine oil.

The oil on the dipstick looks black as it should, the oil on the bottom
of the filler cap was more brown than black, but not milky. The engine
slowly loses coolant over a period of 30 - 45 days. The temp gauge
never climbs, but I know the coolant level has dropped because the
heater starts blowing cold air when the engine as at idle. You only get
warm air if the car is being driven. I never see any leaks, or find any
coolant on the ground. The cooling system was pressure tested. The
mechanic said it held the pressure fine. The vehicle is a 1998 Ford
Windstar with the 3.8L engine, 140,000 miles.

My questions:

1. Is the oil on the bottom of the filler cap telling me the coolant is
in the oil?

2. Would I have to send an oil sample to be analyzed to be absolutely
sure the oil is getting contaminated?

3, If the mechanic determines that coolant has been contaminating the
engine oil for the past few months, would I be tossing money away to
get the van fixed due to damage the coolant may have done to the
engine?

Thanks in advance for any advice

.

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