I'm a national of country XYZ who wants to visit the state ABC for the purpose of tourism. How can I find out if I will need a visa?
NB: this is intended as a canonical/wiki question
One of the most reliable options is to query the Timatic database:
You enter your nationality, your destination and (optionally) other details such as your transit airports and duration of stay. The next page will then show if you need a visa or not, as well as additional requirements:
Airline agents will use the same information to decide if you can be allowed on-board, so if the Timatic website says you're okay to go, you can also be sure the airline won't have any issues.
Online, you can often look to Wikipedia. For example, Visa Requirements for NZ Citizens.
Alternatively, there's sites like VisaHQ, which allows you to specify your passport and the destination country, and find out what you need.
However, if in doubt, the embassy for that country, or their respective website is always the best port of call.
It's also important to note that a visa does not guarantee entrance to a country - ultimately it can be dependent on the airline you use (they need to check if you need evidence of departure, for example in some countries) and also, the border guard on duty when you enter, has some say as to whether you're permitted to enter. Your history (past countries visited, criminal record) can also affect your eligibility.