Usually when you are certified/recertified in ACLS, your instructors will let you know if you are "instructor potential" with an "IP" on your card or a talk after the testing is over. I've seen this with lots of other certifications, too. Teaching is a great way to keep up your certifications and stay updated on a routine basis.
I agree with the "learn one, do one, teach one" method. Works well for me. But beware...when you take the class and your employer pays for it, you will be called to teach it (on top of your usual staffing) when your employer wants you to be there. For BLS this could be a pain in the schedule. You are also required to teach a certain number of classes a year to maintain the instructor level. Make sure you are ready for all that, and Go For It!!