Cactus watering?

i have a red-headed irishman cactus. its in a 4 oz. pot and its about 3 inches tall. the cactus is taking up most of the pot so i cant feel if the soil is moist. how do i know if i need to water it and how much should i water it. i sprayed the soil a little just in case. and is it okay if i get the cactus a little wet? and is it normal for this kind of cactus to have spikes with black tips?