If I fail with carbon monoxide poisoning will people be able to tell?

[John Smith]

Will my face and extremeties be all red or does that just happen after death?

 

[TiredHorse]

Depends on how far along you make it.

Personally, I'd be more worried about the brain damage you might suffer from prolonged hypoxia. Even without that, the headaches and nausea would be pretty awful. CO is only peaceful if it actually works. If you flub it, the side effects are pretty bad.