There are countless ways to run API tests, but a good beginner-friendly way to do so is with the help of a specialized API testing tool. Thankfully, there are many good API testing tools out there including Cartero.

Cartero is an easy-to-use yet comprehensive graphical HTTP client that can be used to test web APIs as well as perform a wide range of HTTP requests to web servers.

Even though Cartero is not the most advanced or feature-packed app of this sort, it has a lot going for it. For starters, it's free, fully open-source, and cross-platform (works on Linux, Windows, and macOS).

According to its developer, the main motivation for Cartero is the fact that there aren't that many native graphical HTTP testing apps out there (that are also free).

Most tools of this sort offer a free version but require payment for more advanced features, while others are not native apps (for instance, there are many API testing tools out there that rely on Electron).

There are very few surprises when it comes to Cartero. The GUI, for example, is very straightforward (and similar to that of other apps of this sort).

The app is compatible with any REST, XML-RPC, and SOAP APIs, and it comes with support for multiple request methods (including attaching body payloads to compatible requests).

In terms of features, the app allows you to load and save plain Git-friendly TOML files (making sure your data is your own), customize and modify request headers and body payloads, as well as work with variable bindings for API keys (and other "secret" information). That's really about it.

Cartero is a prime example of what a good open-source app should be. It looks good, it works as intended, it has enough features to make it useful for a wide range of developers, and it's completely free without compromising security or functionality.