Its so hard to tell someone everything that goes into watching a race. I wouldnt even know where to begin. But I will add something that is of the utmost importance. When watching a race the fisrt time through its natural to watch the horses that you bet on and to be emotional.
Make sure you watch the replay dispassionately. Just watch it without worrying about who you bet, thats over now. Watch the whole race as someone who is trying to learn something.
Another thing thats crucial is not to let who you bet or thought would win to temper your evaluation. I have a friend who constantly makes excuses for every horse he bets who loses. Will say things like "the jockey moved too soon" or too late, etc. If this is going to be you then please don't waste your time even bothering. I will also hear people say things like "the jockey let up on the horse in the stretch" when it has clearly been bashed 20 times with the whip and ridden to the wire. Don't watch the replay to try and make up excuses, as I said, who you bet no longer should be of the slightest concern anymore. Don't watch replays trying to come up with an alibi that doesnt exist for a horse you bet so that you can state your case to friends, etc.
Just watch dispassionately and try to see the reality of what occurred. Thats the point to the whole exercise. You aren't watching to try and create an alibi or reason the horse you bet lost. You are trying to figure out what actually occurred in the race so that the next time you can have the winner. I would point out the English Channel- Cacique race at CD as a prime example of a race where if you watched the replay you could determine that Cacique was hemmed in the whole way behind a moderate pace and came running hard. There wasn't a doubt in my mind whatsoever that he would beat EC the next time they met, none.
Its ok to bet a horse and win and admit afterwards that you got lucky and won with a horse that wasn't the best in the race. Lets face it, it happens all the time and sometimes you bet the best horse and get beat by another who had a better trip. IN those cases people will always state the case of why their horse was best in defeat. Well thats also works in reverse. Sometimes you bet a horse who wins and notice on the replay that another horse or two may have been better. Its not an admission of failure to admit that another horse in the race may have been best, its just common sense.
Try and remove the emotion completely when watching a replay and view it like you would view any science project, void of emotion and trying to learn.