One thing I've heard of is people "leaving the bottle open" next to the bed. That way the ill person can "accidentally" decide for themselves.
The alternative is for patients to refuse food and water until they die.
I'd been thinking about this a lot because my father was diagnosed with a terminal illness 3+ years ago. We'd discussed euthanasia, and he definitely didn't want to spend his final 6-12 months suffering.
Because he didn't die, the docs took another look and realised he'd been misdiagnosed (and yes, we got 2nd & 3rd opinions after the initial diagnosis).