Hi,
Who told you to do those tests?
He should be able to answer all the questions.
Usually there is a reason for tests. And there should be a specification on "what" and "how to" test.
I think there are no "general" rules for high temperature test of capacitors. It depends on the application where the capacitor should be used in.
****
Further informations: I´d look into datasheets especially of "high temperature capacitors" and go through their specifications and test parameters.
For sure there may be other than electrical problems with high temperature capactiors: solderability, mechanical stress, piezo effects... but i don´t know if it relates to your application.
Klaus