You can't really compare by looks. A knowledge of operating pressures is the best thing you can have. Knowing model numbers help too.
I say this because there are regulators for about 15 different types of gas, and they all look very similar. To get a better understanding, I suggest you start with the "how to build a co2 regulator" sticky at the top of this forum. Don't get discouraged by the wealth of information there, either. It's actually much simpler than it looks. If you have any questions about specifics we're all happy to help you out