How do I tell when my cactus needs watering?

The picture is of a succulent not a cactus and therefore needs a different approach. Cacti only need watering about once a month and can be left until the pots and soil is very dry. Succulents need watering slightly more often but I would still wait until the compost has dried out and always water it from the bottom. Stand it in a saucer or bowl of water for a couple of hours and then remove. Place it in a bright or sunny spot out of draughts.