We have two of these carbon monoxide detectors in our house. We received both of them as gifts, but would have gladly paid for them ourselves just to have them in our house. We keep one on each level of our house for total protection.
Thankfully, we have never had to use these to alert us of a CO leak. You can test them to make sure they are working though. They put out a high pitched ring to alert you of danger, or at least that it is working. The sound is different from our fire detectors, so we can tell which one is going off. It is loud enough that it woke my hubby up one night, and he's a deep sleeper! The one we had upstairs was beeping that the batteries were getting low. He heard the sound all the way into our bedroom. Although it was not annoyingly loud, it did get him out of bed to figure out what the sound was.
This detector has a red and green light on it, but they are very small. The green one blinks every once in a while to let you know it is still working. The red one blinks fast when the detector is going off to give you a visual cue as well as an audio one.
The only thing I do not like about this detector is that the lights are so small. Even though they are bright in the dark, during the day they can be hard to see.