The best way to find out if a Component Presentation is Published to the Broker is to query the Broker for it. Optionally you could check the Broker database itself, but since the datamodel is officially not public I don't like to suggest that route.
Using the Content Delivery API you can use the following code to check if a Component Presentation exists in the Broker and get its content from there:
int publicationId = 1;
string componentUri = "tcm:1-2";
string templateUri = "tcm:1-4-32";
// get Component Presentation from Broker
using (ComponentPresentationAssembler cpa = new ComponentPresentationAssembler("tcm:0-0-0", Page))
using (ComponentPresentationFactory cpf = new ComponentPresentationFactory(publicationId))
using (ComponentPresentation cp = cpf.GetComponentPresentation(componentUri, templateUri))
{
if (cp != null)
{
// Component Presentation exists in the Broker
string cUri = string.Format("tcm:{0}-{1}", publicationId, cp.ComponentId);
string tUri = string.Format("tcm:{0}-{1}-32", publicationId, cp.ComponentTemplateId);
// use ComponentPresentationAssembler since cp.Content does not resolve REL
string content = cpa.GetContent(cUri, tUri);
}
}