How I Test Weight Painting on Avatars
Typically most people (at least the people I know) often test their avatar using MMD worlds and such. This involves uploading the avatar which takes time and isn't really ideal. You could try converting an animation onto a character in Blender but that can be time-consuming and wonky, and it's a bit odd to do in Unity normally. I have a way that is significantly easier!
Prerequisite: Install Pumkin's Avatar Tools
Pumkin's Avatar Tools is a Unity Editor Script that adds a ton of useful functionality for avatar creation, I recommend it for everyone, I use it a ton.
I've been calling it Pumpkin's Avatar Tools this entire time and I only realized it now god help me
Using The Pose Editor
Once you have Pumkin's Avatar Tools installed, there should be a new tab at the very top of your screen of the titular name "Pumkin" then open the "Tools" dropdown and select "Pose Editor"
Once the window is open, drag in your avatar into the "Avatar" slot, or select the avatar in your Scene Hierarchy and click "Select from Scene"
You'll now have several sliders you can use to mess with your avatar! This should sufficiently assist in your goal to test weight painting, however you can go a step further and put an animation clip into the "Pose from Animation" field.
You must have usable HUMANOID animations in your project for this to work! You can find some around the internet.
Personally, I have a pack of Fortnite animations that I've had on my computer for a couple years now, so that's what I'll use to demonstrate.
With this, you can change the animation on the fly, and even use it in playmode as shown in the GIF above! It'll also automatically loop.
Once finished, you can scroll to the bottom of the window to reset to the avatar's original pose.