Blaine Bettinger runs the Shared Centimorgan Project in which he's collected thousands of data points for amount of DNA shared between people with known relationships. He can give a range of DNA which has been observed to be shared for people with specific known relationships. If you have a new DNA match, you can see how much DNA you share with that person and use Blaine's chart to identify potential relationships that are possible with that person.
|Shared cM Project. From http://thegeneticgenealogist.com/wp-content/uploads/2016/06/Shared-cM-Project-Version-2-UPDATED-1.pdf. Used with permission.|