Carbon Monoxide has no smell to it so without a carbon monoxide detector you will not know if carbon monoxide is in your house until you or someone becomes sick because of it.
If you're smelling a rotten egg smell in your home and you have natural gas in your home then you likely have a natural gas leak.
Natural Gas is actually odorless but when natural gas is extracted they put a substance called mercaptan into the natural gas which emits an odor like rotten egg smell so it makes it easy to detect a natural gas leak.
If you're smelling a rotten egg smell in your home you should get away from the home and contact the natural gas company to come out and check for leaks.