As I am currently studying violin, I can only share knowledge about it. An advanced musician will know how to attune his instrument just by the sound, but for a beginner, I think this kind of app below helps. The arrow must be right above E, A, D and G; it depends on the string.
https://www.youtube.com/watch?v=KWXqI-TcTeo (https://www.youtube.com/watch?v=KWXqI-TcTeo)
Please feel free to share your musical knowledge and failed or not attempts at playing something.
Dearly,
Tsukiyoarts