Tools needed: 

• Appropriate size screwdriver for lengthening/shortening your bridge saddles
• An accurate tuner (no headstock tuners)

Tune your bass as accurately as you can. Hit the open G string and observe your tuner. It should be spot on. Now fret the G string at the 12th fret and strike a (gentle) note. The fretted note should also be in tune. If the fretted note is sharp, the string is too short. If it is flat, the string is too long. Adjust each saddle accordingly. (Might want to loosen the string tension before lengthening a string. Loosening should be ok under string tension.)