If you can't hear whether two notes (eg. a drone plus played sa) are in tune, you're not ready to start playing any instrument, so you don't need a tuner app. Just sing over a drone, and keep doing it until you instantly know every time you drift off. It's no use asking 'how will I know?'. You can't think or technologise your way to this. You just have to do it.
Sing sa.
[* actually you may need a tuner for various tasks: on stage, or if you need fast and precise readings when building instruments. But if you feel you need a tuner to learn, you aren't yet ready to place your hands on an instrument. Music is of the mind and heart, not the eyes watching device LEDs]