I don't think you can ever say 'oh, they did such-and-such, so they can't have been suicidal.' But some people can behave apparently normally right up until they take their own life. Some can seem happier than they have for ages, because they have made up their mind. Some try to complete normal tasks so as not to leave things 'undone' when they go - they can wash up, tidy the house, walk the dog and lay out the tea things, for example. And relatives will use this as a reason to disbelieve suicide.