I absolutely agree with this and with HAF’s articulate reply.
Here’s video of my filly when she showed us her gaits at three weeks. This was when I knew what I had. Although there’s no canter here, her canter as a foal was always balanced. She changed leads cleanly and effortlessly, and still does.
http://www.youtube.com/watch?v=E8NXT6-9Iq4&NR=1
Now, I am definitely a hindleg kind of gal. I focus on its quickness and reach more than the front end. BUT…there’s no single indicator of adult movement, IMHO. Rather, it’s a combination of things and you have to look at the whole picture.
You have to study lots of foals and go to lots of inspections, farms, and shows. See as many as you can at 3 days, 3 weeks, and 3 months. Alexandra’s suggestion about observing with a friend is wonderful advice, since no two breeders seem to ever gravitate to the same things.
Sorry I don’t have “adult” footage of the above filly. There are some still photos on her webpage:
http://www.runningwaterwbs.com/Raleska.html
I hope to get footage at her inspection in August, so you’ll have take my word for it that the quick hindleg, lovely balance, and swing through her body are still there. Or you can take the judges’ word for it! She’s been GAIG Region 8 Filly Champion two years in a row.