When you are looking for a weanling or a yearling you need to look closely at both parents to see what you may end up with. You also should select an experienced, reputable breeder that can tell you what they think you can expect from the foal you are considering, the other offspring the mare has produced, what they moved like, what their temperaments were like and how competitive they were in the sport that you are thinking of using them in.
Needless to say (But worth mentioning anyhow) you also need a breeder that is interested in finding the correct match for the foal they are selling, both for the horses sake as well as yours. Not just someone interested in selling whatever foal they have, to whatever buyer shows up.
Also check what other successful foals the sellers has produced for the sport you wish to compete in. Believe it or not this does count quite a bit. If someone has bred their mare because they "did not know what else to do with her" (and lots of people make this mistake! )this very, very rarely produces a foal that will be desirable. You would have about the same odds of winning the lottery.
Take this Sandro Hit filly.
She has very much her dam's looks and her sire's movement (Video at http://www.youtube.com/watch?v=kpYm0k2GmqM
) at 3 months old. By the time she is 6 months to a year old this look and movement may "disappear" for a while due to growth changes however I can guarantee that this filly will have this spectacular movement, a beautiful look, and conformation as a 4 year old and it will continue (barring any major mishaps) throughout her life. She will also produce similarly lovely foals of her own. I also know the mare temperament and can tell you exactly the type owner that this filly will perform best for.
Another case in point, I have a Contucci mare (6 years now) that I bred. I watched her dam compete and win the overall 4th level championship at Dressage at Devon with her amateur rider. She was also Grand Champion(the first mare to beat all of the stallions) of the Dressage at Devon breed show.
When this foal was 3 months old she was lovely with 3 outstanding gaits but by the time she was a yearling Alpo would not have put her in a can she was so ugly. I kid you not ! She had a huge head and a skinny neck that looked like it came out of her knees. Her gaits were so bad that I was not sure what she would ever do with her life. Even at 3 she would canter and seem to forget that she even had 4 legs much less where to put them! She could not stand to be touched and was very timid until about her 3rd birthday.
A top breeder looked at her with me one day when she was just being started under saddle as a 4 yr old (she was so tall we had to wait!)and I said "what do you think I can do with that one" and she looked at me and had no idea what to say. She is an amazing young mare now. Pictures to be found on my website www.RiversideEquestrian.Org
(sales page - click on Carrara), lovely gaits and the sweetest most affectionate of dispositions. She is quiet and comfortable, has 3 super, expensive gaits with impressive looks and tons of "look at me presence" for the dressage arena (very similar to her dam!)