I somehow sense when something's really wrong. The last time I went was when my bowel was blocked. It perforated soon after, but at the time I decided I needed to go it was just blocked. The pain was not the worst pain I've ever had. I had no output from my stoma, but only for half a day or so at that point, which isn't unusual for me. I'm usually sceptical about it when people say their body is trying to tell them something, but I can't think of another reason why I knew I had to go. I was sceptical about myself too, so it took my parents urging me to go before I agreed. What's more usually the case is that my mum urges me and I refuse (and I've been correct to refuse). So I suppose I would say that, if you're like me (who hates hospitals), if you need to go, you'll know.
When I got to the hospital, they admitted me within minutes (the sign said the waiting time was four hours), and within about an hour or so of arriving I'd seen a doctor, been given painkillers, had an NG tube inserted to drain my stomach, had an x-ray and seen a surgeon. When you really are an emergency, they are the place to go. They are specialists: they specialise in getting you stabilised as quickly as possible and diagnosing emergency medical problems. With Crohn's, these are often bowel blockages and perforations and dehydration.
Generally, abdominal pain that is constant is more likely to be serious than cramps. A fever is often an indication of something serious. Being unable to keep anything down, and/or excessive diarrhoea to the point of dehydration symptoms requires immediate treatment. Vomiting blood or stool, passing excessive blood with stool, pain that's accompanied with inability to have a bowel movement for many days, or severe watery diarrhoea are also reasons to go to the emergency department.
You can also go for painkillers. Going for pain relief is a valid reason. They should try to make you comfortable until you're able to see specialists as an out-patient. The emergency department is also there to give you peace of mind if you are unsure whether your symptoms indicate an emergency or not.