What (if any) temperature differential should there be between the attic temperature and the exterior ambient temperature in order to consider adequate or inadequate ventilation exists. Is there any given rule of thumb?
Your best bet is to do a rough calculation of the fascia vent area and the square footage of the attic. I forget what the required ratio of vent area to attic area is.
Be sure to look not only for incoming air vents, also look for vents to exhaust the air. They should be roughly the same total amount of area.
Yes, but what about determining if the vents are working as intended?
When it’s hotter than blue blazes (how ever hot that is 
) in a vented attic, how hot is too hot compared to extenal temps. Heat is as much or more detrimental to the roof covering as water.
Usually, I like to see no more than a 10 to 15 degree differential between attic and outside air temp.
Good free flowing air helps keep ice off our roofs and cools the attic helping to insure the roofing’s “normal” lifespan.
There are factors other than the ventilation that determine the heat in the attic. One very simply is the color of the shingles. Dark shingles will hold and radiate the heat through the attic on a sunny day much more than light ones.
If the attic has ample ventilation, and the vents are not obstructed by insulation, crushed baffles, etc., they are probably working as intended. I’ve been in attics that were 130 degrees on a sunny day, and could feel the air moving past me from the vents.
Moisture indications are more usually the signs of improper or inadequate ventilation.
There SHOULD be no difference in temp.
But how often do you see that?
I’ve never heard of a formula for temp differential.
The closer to the outside temp, the better.
If you’re looking for a general rule of thumb for attic ventilation:
1 sq ft of net free air per 150 sq ft of attic if no vapor barrier
1 sq ft of net free air per 300 sq ft of attic if vapor barrier exists
This applies for crawls, too, although crawls should always have a vapor barrier of some sort.
Even if these requirements are met, recommend additional if excessive heat/abnormal condensation is detected.