I only can imagine these options: just CTB without any kind of sign, so it'll be abrupt and sudden; giving subtle hints of what you're gonna do - talking about suicidal people, the hopelessness for the future, how life is cruel and nonsense for some - so they'll realize somehow you're not fine; and, the final stage, openly saying you're preparing to die, what may cause panic and intervention.
P.S : I really wish humankind achieves a point where discussions about suicide can be honest and boundless, not trying to convince the other of how wrong and ill they are for desiring death.