Symptoms of coccidiosis are lethargy, sitting fluffed up, isolating themselves, going off food/water, runny or mucousy or bloody droppings. Sometimes they can have balance or coordination problems. It's correct that only some strains will present with blood. Any time a chick or young bird displays any of the symptoms then coccidiosis should be considered, and if in doubt, it should be treated. Corid is very safe and will do no harm even if it's something else. Other times it should be considered, even with older birds, is if new birds have been brought into the flock (they may bring in a new strain), birds are moved to new ground (may expose them to a new strain), or if you have had a period of wet weather where the ground stays moist for a period of time (wet, warm conditions contribute to what is called a 'coccidia bloom', where the numbers of available oocysts in the environment can be increased). Some strains are more virulent than others and can progress and kill very quickly, others are less virulent and you may have more time. All of them can kill if left untreated, and the longer before treatment is started the more damage will be done to the digestive tract.
You might be interested in this thread, video in post #1 that is very informative on what the coccidia do to the digestive tract, how common it is, etc. It's worth watching.
https://www.backyardchickens.com/threads/coccidiosis-video-worth-watching.1262022/#post-20259051
*EDIT* For the dirt in the brooder, I use a large plastic plant saucer. I dump and clean it daily and put in new dirt. The saucer makes it easy to dump and clean. They make a mess, we call it the 'dirt party' because they love it!
