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+Yes, you absolutely can (and should) verify this locally to save time on CI runs.
+
+Since you are running Arch Linux (Rolling Release / Qt 6) but need to verify a build for Ubuntu 22.04 (Qt 5.15), you typically don't "cross-compile" in the traditional sense (which usually means building for a different CPU architecture like ARM). Instead, you **containerize**.
+
+The best way to verify your fixes is to run a **Docker** container that mirrors the Ubuntu 22.04 CI environment exactly.
+
+### **Option 1: Using Docker (Recommended)**
+
+This allows you to compile against the exact Qt 5 versions used in Ubuntu 22.04 without messing up your host Arch system.
+
+**1. Start an Ubuntu 22.04 container mounting your current source code:**
+Run this from your feather repository root:
+
+```bash
+# -v mounts your current directory to /feather inside the container
+docker run -it --rm -v $(pwd):/feather ubuntu:22.04 bash
+
+```
+
+**2. Inside the container, install the dependencies:**
+(I copied this list directly from your `.github/workflows/build.yml` file for the Ubuntu 22 job)
+
+```bash
+apt update
+apt -y install git cmake build-essential ccache libssl-dev libunbound-dev \
+libboost-all-dev libqrencode-dev qtbase5-dev libqt5svg5-dev libqt5websockets5-dev \
+qtmultimedia5-dev libzip-dev libsodium-dev libgcrypt20-dev libx11-xcb-dev \
+protobuf-compiler libprotobuf-dev libhidapi-dev libusb-dev libusb-1.0-0-dev
+
+```
+
+**3. Run the build:**
+
+```bash
+cd /feather
+mkdir build-compat-test
+cd build-compat-test
+# This will trigger your new auto-detection logic
+cmake ..
+# Compile (use -j to speed it up)
+make -j$(nproc)
+
+```
+
+If this succeeds, your CI job will succeed.
+
+---
+
+### **Option 2: Installing Qt 5 on Arch (Native Side-by-Side)**
+
+Arch Linux provides Qt 5 packages that can coexist with Qt 6. You can force CMake to use them.
+
+**1. Install Qt 5 base and modules:**
+
+```bash
+sudo pacman -S qt5-base qt5-svg qt5-wayland
+yay -S qt5-websockets qt5-multimedia
+```
+
+**2. Force CMake to use Qt 5:**
+You need to tell CMake to prefer the Qt 5 installation. You can do this by explicitly setting the path or just hoping your `find_package(Qt6 ... QUIET)` logic fails correctly (it might not if Qt 6 is also installed).
+
+To verify the **fallback path** specifically, run cmake with the variable to disable Qt 6:
+
+```bash
+mkdir build-qt5
+cd build-qt5
+cmake -DCMAKE_DISABLE_FIND_PACKAGE_Qt6=TRUE ..
+make -j$(nproc)
+
+```
+
+**Why Option 1 is better:**
+Option 2 tests "Qt 5 on Arch" (which is Qt 5.15.12+). Option 1 tests "Qt 5 on Ubuntu 22.04" (which might be an older patch release of 5.15). The Docker method is the only way to guarantee you haven't used a symbol that exists in Arch's Qt 5 but is missing in Ubuntu's Qt 5.