My vet says that the best indication is to look at the wrist joint just above the front paws. The larger that joint, the larger the dog will be full grown. Apparently that joint does not change in size very much from a pup to a full grown dog.
You can sort of see it in this series:
Seven weeks
Three months
Seven months
Just under one year
18 months. Here she's clearly got her full growth and will only fill out a little. You can see that her wrist joints were never "knobby" and even as a puppy were only slightly larger in proportion to the leg than they are when she is full grown.
And that tallies with her being a smallish (65 pound) Lab. That's the bottom of the range of the standard. My Chocolate is way undersized (her wrist joints were always small, too).
Shelley at about 16 weeks