Though not perfect,
www.musictheory.net has some great ear training tools...I say they are not perfect, because the cheap midi sounds, are very different from hearing say a bass or a piano, but, that's not to say you can't learn something from those trainers.
that is if you don't have a friend to help you.
The other thing I practice doing, is singing my solos, and every thing I play, really. again helps me find that connection to the bass, that I find helps my ear greatly.