Diastasis recti is more likely to turn up in women who have given birth multiple times already, due to more frequent episodes of stretching. If your separated abdominal muscles do not move closer together again and then start closing within about six weeks after you have given birth to your baby, you may need physiotherapy to correct it. You can feel whether you have separated abdominal muscles by tensing up the muscles, and trying to put some fingers in the middle. Women with diastasis recti during the postpartum period will be clearly able to feel two definite "sides" to their abdominal muscles, with a cavity in the middle.
Keep in mind that doing ab crunches with diastasis recti is not advisable. Abdominal exercises might have the opposite effect to your muscles going back to normal which is presumably what you wanted and instead keep the muscles permanently separated. Talk to your healthcare provider before resuming work outs after you have had a baby. The six week postpartum checkup is a good time to ask questions.