The cap keeps the coolant under pressure so that the liquid is under pressure. If you think back to science class you'll recall the when a fluid is under pressure the boiling point of that fluid is increased. Therefore by keeping the coolant under pressure the temperature the coolant can reach before it boils is higher.
Which is good because we never want our coolant to boil! All kinds of bad things could happen.
To test this I imagine you would put a pressure gauge were the radiator cap goes. I can't remember the pressure you are looking for though. It would be atlease a couple psi.
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Driving: 1998 F-150
Far way in Edmonton==>
1995 Mustang GT