Normally you won't have to take a guinea pig’s temperature. Keep them warm, but don't let them overheat. Simple as that.
A Guinea Pig keeping cool
Guinea pigs don't have sweat glands like humans, so they will need plenty of shade during the hotter months, so that they don’t overheat. In the summer, if you don’t have an Eglu Go Guinea Pig Hutch where the guinea pigs can move in between their house and their run at will, then you’ll need to provide a shaded area for them to keep out of the sun, as they’re liable to overheat and get sunburn, or even heatstroke.
If you need to take your guinea pig’s temperature, you will have to use a pet rectal thermometer. This is a delicate business, so get your vet to show you how to do it. Guinea pigs have very small anuses, so you’ll need a special thermometer and to use lubrication. Your pet's temperature should be between 98.96 and 103.1 degrees fahrenheit.