Guitar Tuner
If you haven't got a guitar tuner handy then this is the next best thing! Standard tuning (EADGBE) is the default but if you're playing in an alternative tuning such as drop D or DADGAD for example - then choose what you want from the right hand button.
There are two ways to use this tuner.
As this is a classical guitar site then you probably won't have a pick-up on your guitar. If you have a microphone plugged into your computer or the built in mic activated on your phone then you can see on the swing-o-meter when you're exactly in tune. But if not, then the old-fashioned way of tuning by ear can be done by tapping on the string on the tuner which will give out an audible sound.