Question

How can you tell if someone is planning on commiting suicide?

Answer 1

Ask them — seriously.
Do they have a plan? Do they have access to resources that could carry out their plan?
If they have been severally depressed and then suddenly seem to improve this could be a sign they have decided to attempt suicide.

Answer 2

Them becoming either distant or angry if you ask them certain things, the way they talk eg. saying the world would be better with out them around.