Here is a super helpful tool for testing your fonts. I use this tool all the time with all of my font projects. It allows testing in a multitude of different ways, various layout formats and text compositions as well as in many different languages.

you can find it here:

Font Testing Page

The Font Testing Page is a tool primarily intended for type designers to 'Drag and Drop' and quickly test their fonts into a browser. It can also be used by Type and Art Directors, Graphic Designers, Teachers and Students interested in seeing how a typeface works on the web. The tool is a open-source shared community effort on GitHub.

For more detailed information about the Font Testing Page tool and its usage please visit GitHub.

Here is another helpful type webtool called FontDrop! — Simple and easy way to view the content of font files. It also provides very basic testing capabilities. Nowhere near the level of depth "Font Testing Page" provides. But helpful for a quick and simple way to check what your font file can offer, especially if you are not familiar with the font editors or command line tools that professional type designers or font engineers would use. Simply drag and drop your OpenType or TrueType font files onto this website to see what’s inside them. 

https://fontdrop.info/

For a more detailed description of the tool you can visit this page on Viktor Nübel's (the original creator) website.

[Kerning] The Typographic Art of the Space Between Letters

Kerning is the spacing between pairs of letters. A major important part of type design and essential in the development of a professional typeface. Its a necessary task when working on a professional typeface, something you can not and should not skip, or neglect. While the process is part science and part personal preference, there are some guidelines and basic rules to ensure things remain consistent. Good kerning doesn't come easy, and it requires a lot of practice. In the end this makes or breaks your typeface.

Below are some links to various helpful articles and shared knowledge on the general concept and the art of kerning:
adobe.com/kerning
microsoft.com/what is kerning
canva.com/kerning
totallytype.com/kerning
icons8.com/ultimate guide to kerning
fabrikbrands.com/what is kerning in typography
sanserifs.com/the art of kerning: perfecting letter spacing

Kerning in FontStruct follows the same general concept guidelines and basic rules that apply, but the the process is very different from other more professional type design tools. Its far more basic, less streamlined and more hands on manual labour. Large charactersets therefor become very time consuming tasks when it comes down to kerning within the FontStruct editor.
The best and quickest way to execute this task thoroughly is by way of working with strings of character combinations/pairs. And to make sure you work consistently, it's recommended that longer strings are chopped up into workable chunks that isolate into seporate smaller portions for each individual character.
But even then it remains a painstakingly labour intensive task. And this is only the kerning process by itself.
Making preperations for this in order to set things up correctly also requires substantial additional labour.

There are some tools to consider that can help streamline and speed up the preparations for kerning in the FontStruct editor.
• Combination Generator
• Remove Line Breaks
• Remove All Whitespace

@BWM: Yes true, but using this trick all levels of zoom in the preview widget look good, regardless of the internal font structure!

Here is another comparison when zoomed in (top) and no zoom (bottom).