There are tuning apps available for free, and some for a small fee.
Play a note, and the app will indicate whether you are in tune, or whether you are sharp or flat.
Here are four apps (there are also others to choose from)
BandMate Chromatic Tuner
Tuner Lite by Piascore
Tuning a String Instrument:
See the instructions for Tuning a String Instrument elsewhere on the web page