How do I test for adequate attic ventilation?

I was looking at this sight and got curious about my attic ventilation.
http://www.airvent.com/homeowner/pro...of-specs.shtml

I was looking at the roof vents and there are only three for a 2500 sqft
single story house. The vents that I have looks like these
http://www.airvent.com/homeowner/pro...ts-sqMetal.jpg

I estimate the free area at 190 sqin free area total for the three. There
are no ridgevents or gable vents to help out. There is a continuous soffit
vent around the eave of the house and I calculate the area as 2000 sqin
total. More than enough

I suspect the there is inadequate ventilation on the roof vent side and I'd
like to test for this.

Is temperature the big deternining factor.

This is in Houston, TX where HEAT is a big problem.