To incriminate a person for a capital sin requires a warning, to distinguish between intentional and unintentional transgression (שוגג או מזיד, Makot 6).
A person should be warned that the action he is about to perform constitutes a transgression and the punishment for that transgression is [one of the four types of] death.
To be sure the person understands the warning and carries the action intentionally he needs to reply with a certain wording ("שעל מנת כן אני עושה" - "that's exactly my intention").
If the person stays quiet or says something unclear he's not considered Mezid.
What does it mean "שעל מנת כן אני עושה" - intention to carry out despite the punishment or intention to be killed?
What is the general formula that can ensure he is Mezid? Are there special "stop-words"?