Google Fonts' CSS is lying about its WOFF2 files: How can I know that in runtime/while rendering?

Question

Context

I'm working on an app that needs to dynamically analyze and apply fonts from Google Fonts. Part of that consists of reading the unicode-range CSS property to: first, get to know which characters a given font includes (this is the actual pain point/lie); second, render those characters with that font.

The Problem

A given unicode-range might list code points that are not included/covered with glyphs by its respective WOFF2 files.

I did realize that by a) visually testing it: characters will render with a given fallback font (usually serif if there's no fallback); or b) by requesting a font specifying included but actually missing characters as the text value on its URL: in such a case Google Fonts use to return CSS with broken WOFF2 URLs.

The Question(s)

1. Is there any JavaScript/CSS API we can use to know when a character is rendered by a fallback font?
2. Is Google Fonts automating its CSS building process in such a way that unicode-range(s) are likely to always include some missing code points?

Example

For the sake of simplicity I'm not going to bring here all the code I came up with to automate the following process because it's out of scope.

Roboto includes the following characters on its cyrillic-ext subset:

Ԕ ԕ Ԗ ԗ Ԙ ԙ Ԛ ԛ Ԝ ԝ Ԟ ԟ Ԡ ԡ
Ԣ ԣ Ԥ ԥ Ԧ ԧ Ԩ ԩ Ԫ ԫ Ԭ ԭ Ԯ ԯ

Here we have those characters (actually not) rendering with Roboto:

/* cyrillic-ext */

@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 400;
  font-display: swap;
  src: url(https://fonts.gstatic.com/s/roboto/v30/KFOmCnqEu92Fr1Mu72xKOzY.woff2) format('woff2');
  unicode-range: U+0460-052F, U+1C80-1C88, U+20B4, U+2DE0-2DFF, U+A640-A69F, U+FE2E-FE2F;
}

.fallback--serif {
  font-family: Roboto, serif;
}

.fallback--sans-serif {
  font-family: Roboto, sans-serif;
}

.fallback--monospace {
  font-family: Roboto, monospace;
}

<p class="fallback--serif">
  <code>serif</code> fallback<br/> Ԕ ԕ Ԗ ԗ Ԙ ԙ Ԛ ԛ Ԝ ԝ Ԟ ԟ Ԡ ԡ<br/> Ԣ ԣ Ԥ ԥ Ԧ ԧ Ԩ ԩ Ԫ ԫ Ԭ ԭ Ԯ ԯ
</p>
<p class="fallback--sans-serif">
  <code>sans-serif</code> fallback<br/> Ԕ ԕ Ԗ ԗ Ԙ ԙ Ԛ ԛ Ԝ ԝ Ԟ ԟ Ԡ ԡ<br/> Ԣ ԣ Ԥ ԥ Ԧ ԧ Ԩ ԩ Ԫ ԫ Ԭ ԭ Ԯ ԯ
</p>
<p class="fallback--monospace">
  <code>monospace</code> fallback<br/> Ԕ ԕ Ԗ ԗ Ԙ ԙ Ԛ ԛ Ԝ ԝ Ԟ ԟ Ԡ ԡ<br/> Ԣ ԣ Ԥ ԥ Ԧ ԧ Ԩ ԩ Ԫ ԫ Ԭ ԭ Ԯ ԯ
</p>