A Diastasis is a separation of two body parts that are supposed to be together, in this case, it is a separation of the Rectus Abdominis Muscles. 

A Rec-check is a great way to determine the length, width, and depth of the separation and how much damage there is to the connective tissue. This is usually done by feeling the space between the abdominals while doing a sit-up. Two fingers or more is considered a Diastasis of the Rectus Abdominis muscles. It is great to keep a record of this to assess your clients progress. From my own experience, the space can even become smaller in just one session.

Below is a small video from my Pregnancy and Postnatal course on how to do a Rec Check.