TO,
It takes a lot of practice to spot the difference.
For example there are a couple of ways to look at stride length, but I must emphasize firstly, that not all good striders have the right engine behind them.
Sometimes they are too slow and basically plodders.
When being paraded, look at the reach of the front legs, the bobbing of the head as the horse takes it's stride. Is the horse taking lots of little steps with the head rapidly bobbing in short sequence.
Or is the horse taking long strides and reaching with the head.
Here's Black Caviar's half-brother.
http://youtu.be/U3xpBvzRMdw
You'll note a couple of things here.
The horse is leading the handler, not vice versa
The handler is merely steering.
The action of the front legs is fluid with long slow strides covering a lot of ground with each stride, so much so, they had to remove the other horse from the ring, as he was catching up!
Whether or not he also has the speed is another issue, but the basics are there including the breeding.