The vast majority of heifer to bull twins are free martins, ie missing some or all of the important parts needed for breeding - there are several ways to determine if she is a free martin - there is the "test tube test", or you can wait until she is of sufficient size and either palpate or ultrasound her. There is a blood test that looks for male cells in the heifers blood, but all that tells you is that there was mixture of blood btwn the heifer and bull calf in utero - doesn't tell you anything about what she does or doesn't have.