You can execute systemctl --user --failed to check for failed systemd user units. But let's face it: It's inconvenient and you'll probably miss failures. Better use desktop notifications.
~/.config/systemd/user/service.d/user-failure-notification.conf
Add a OnFailure handler to all user units.
[Unit]
OnFailure=user-failure-notification@%n
~/.config/systemd/user/user-failure-notification@.service
Add a template service unit which sends the notifications. The instance variable %i is replaced by the calling unit.
[Unit]
Description=Send a notification about a failed systemd user unit
[Service]
ExecStart=/usr/bin/notify-send -u critical -a systemd "Failed unit: %i"
Activate
systemctl --user daemon-reload
Check
Run systemctl --user cat $unit to verify the config:
$ systemctl --user cat pipewire.service
# /usr/lib/systemd/user/pipewire.service
[Unit]
Description=PipeWire Multimedia Service
Requires=pipewire.socket
ConditionUser=!root
[Service]
LockPersonality=yes
MemoryDenyWriteExecute=yes
NoNewPrivileges=yes
RestrictNamespaces=yes
SystemCallArchitectures=native
SystemCallFilter=@system-service
Type=simple
ExecStart=/usr/bin/pipewire
Restart=on-failure
Slice=session.slice
[Install]
Also=pipewire.socket
WantedBy=default.target
# /home/$user/.config/systemd/user/service.d/user-failure-notification.conf
[Unit]
OnFailure=user-failure-notification@%n
