Use New Braunfels' General Reference Map to check whether an address is inside city limits, then its City Limits and ETJ Map to check whether an address outside those limits falls in the ETJ.
The city points residents to the first map on its New to NB page and the second on its Planning Services page. Together, they answer two questions that a New Braunfels mailing address cannot settle.
For a Comal County address outside New Braunfels city limits, an ETJ designation does not by itself make the occupant a city property taxpayer or eligible for city resident services. The city's resident guide says people in the ETJ do not pay New Braunfels city property taxes. It also cautions that having New Braunfels Utilities as a provider does not establish city residency.
Check the address in two steps
First, enter the street address in the General Reference Map linked from New to NB. The city presents that map as the way to verify whether the address is inside its limits. A postal address alone is unreliable for this purpose: the city explains that mail delivery, rather than the municipal boundary, determines the mailing city.