Electronic tuner
The easiest and most accurate way to tune your bass is by using an electronic tuner. An electronic tuner allows you tune each string individually to the tuner, by indicating whether the notes are sharp (too high) or flat (too low).you can plug your bass directly into the tuner using the same lead that plugs into your amplifier
Relative Tunning
Here's another way to tune your bass. it is less accurate,as it assumes that your starting string is in tune and it is harder for the ear to hear the low pitch of the bass as well as a tuner can-but this method works when there are no other alternatives. it's also a good way to check your tunning
start with your open G string.If there is an accurate G available,use it; otherwise,assume the G string in tuned to the correct pitch.
Play the D string at the 5th fret,and see if that note matches that open G. Tune the D string up or down untill the two notes match.