There are two primary spots to measure the gravity. First is after you collected all of your wort and it has blended sufficiently. This is your pre-boil and will tell you how much sugar you got out of the grains.
The pre-boil is also where you would want to determine how much DME to add. Determine your gravity and subtract that from your expected. DME is about 40ppg. If you have 8 gallons of pre-boil volume, and your are 5 points short, you can add 1lb of DME to get you back up there.
Second is after the boil. This is no different from extract brewing.
There are other spots to measure, before lautering, mid boil, while sparging. I don't do any of those, but plenty of others do.