By now most people accept that the only way to know for sure if a person is suicidal or not is to ask them clearly and directly.
It is often helpful, for both people, if the questioner includes some context, some justification for asking.
The questioner can include the things he/she has seen, heard, sensed, or learned about
e.g. “Joe I` ve noticed you ve lost interest in your appearance, I ve heard you say there`s no point in anything, I ve sensed you`re struggling with even simple things and I know Louise says it`s over for good this time”
With all this I`m wondering…. are you thinking about suicide?”