The above link is an excellent PDF showing the various types of tongue ties (including the maxillary frenulum, which most of my children had/have!) and explains how these issues can impact breastfeeding. While diagnosing a tongue tie is outside my scope as a doula, it's something I've learned about in the Managing Breastfeeding Complications course and through personal experience - our fourth child had significant nursing issues. A lactation consultant and pediatrician both checked her and determined she had the maxillary frenulum but not the tongue tie. Her latching issues cause both of us a great deal of stress & tears (and caused me a lot of pain, bleeding, and recurring mastitis!) Tongue ties can be clipped (or lasered) and in most cases moms report an almost immediate difference in how nursing feels - the baby is able to latch better, drain the breast more effectively, and fill up their tummy! A doctor can help determine if treatment is necessary or not depending on the severity of the tie and the baby's nursing ability.
As a doula it's important that we be aware of potential nursing complications such as this so we can help connect parents to help. I have a resource list to include local lactation consultants and pediatricians who can diagnose and treat issues like tongue tie. Viewing the PDF above can also help us know better when we need to make those referrals!