I will add to mavs advice.

Look at whether the horse fond trouble during the race. Ie. blocked for a run. I find that it is helpful to look mainly at horses who find trouble/interference in the closing stages rather than at the start of the race. Unless the horse completely misses the start most early errors can be corrected throughout the running of the race (depending on jockey skill).

One thing that i find most important though in reards to interference is to look at whether such interference is likely to be repeated in future races. This is not easy. But some horses, mainly backmarkers, have a knack of finding trouble.

Could go on but i won;t.