I hold the bobbin case in one hand, then take the thread coming out of it and hang it from the thread with the other hand. The thread should not come out more unless it is jerked. If it has to be jerked hard for the thread to come out, the tension is too tight. If you can't hold the bobbin case by the thread without more thread coming out, the tension is too loose.
Once you are sure the bobbin tension is approximately right, you will have to readjust your top tension. It is probably too tight now. Test the tension balance by using your widest zigzag stitch. This will show a tension imbalance more clearly than a straight stitch.
Also check to make sure that the thread coming off the spool is not getting hung up on the spool. You may also need to clean between the upper tension discs; a tiny piece of thread caught in there will cause the tension to be too high and the thread to break while sewing.