As other posters ... such as the learned Blackthroatedwind ... have and will tell you ... it's a complex subject ... but ...

... to boil it down to the two simplest factors ... they are ...

[1] Watch to see how well a horse quickens his pace when the jockey asks him to. If he accelerates quickly and moves past rivals easily ... and/or lengthens quickly if on the front end ... that's a very good sign. If he only inches up to rivals ... or fails to lengthen out ... that's bad news.

[2] Make note of the horse's time for the final furlong. Did he do it easily and in fast time? Very good. Did he have to work hard and still come home slowly? Very bad.

If a horse does well by these two criteria ... he's likely to come back and run another good race ... and vice reversa.