some manufacters have added the thermal sensors to some video cards, if yours has or has not them youll need a program to read the temperatures anyway; the program should come bundled in the video cards softwares. i know the GTS made by ASUS has the thermal sensors. otherwise you should add them by yourslef, which must be a very carefull mod.
i think that if the CD or the manual of the card has nothing about reading the temperatures, yours might not have the option.
when i need to check the temp on my video card i use just to put my finger very carefully above the heatsink. its not quite precise but it will let me know at least if the lockups r due to heat or not.
about the freq, this is something youll really need to test out with the card, hardass. because the limitations differ from card to card, even when both r from the same manufacter and model. this kind of thing depends a lot on the equipment itslef, like memory modules etc.