Starlight, in answer to your question, you know when something else is wrong because when you go into comfort them they don't stop crying.
CC is about reassurance. You put the baby down, it cries. You comfort the baby. Put the baby down, it cries, give it a minute, comfort the baby, continue for however long it takes. Its a process, which might take a long time, it might be over in 20 minutes. If the baby wakes in the middle of the night, you don't necessarily leave him or her to self-settle, you'll listen to the cries, maybe wait a minute or two, use your
instinct and common sense.
How your baby gets to sleep is such a small part of parenting, and won't be compromised by the love, cuddles, reassurance and sound sleep your baby enjoys for the other 23 or so hours of the day.
There's no right or wrong. I've used CC, I've done co-sleeping. Its all worked because it had to. But I did find the more I had to stick with a routine (with the 3 eldest DCs) the more relaxed I was with DC4.