"Not that I can tell" = "Not in any way that I can understand/discern". Sometimes it also just means "I'm unaware/I don't think so".
You are aware of how some computer application works. Someone asks you, "Can it do this as well ?"
If you don't think it does, but you are not sure then you could say: "Not that I can tell".
ie. I'm not aware that this application can do it. You are also implying that you don't think it can do it.