The spotlight method works well. If you doubt how close you are, though, slow down (as has been previously said) because depending on your rocket, even at full thrust, if you're really descending fast you may not be able to slow down fast enough.
If you're in a manned capsule, you also have a landing radar inside the cockpit, which will tell you your altitude above terrain. It's pretty nerve-wracking to use though, because you have to use the inside view.
RocketBlam's post in Landing on dark side of planet? was marked as the answer