By which I mean the first 2 weeks; during which time the following non-specific symptoms may occur. Sore throat, or hoareseness of voice, or dry tickly cough, or blocked nose, or sneezing, or runny nose, or slight fever, or general debility, or any combination of these.

Typical whooping cough, with nothing except choking attacks of coughing, vomiting or retching afterwards, and sometimes a whoop after a few seconds of breath-holding, usually takes about 10-14 days to start to develop. 

The main message to note is that during this first phase the symptoms are very variable from person to person. Early symptoms can last for a couple of weeks and during that time there is nothing to indicate that it is whooping cough. A culture test (too many false negatives), or PCR test (much more accurate), should be positive in this phase.  Read lab-test page.

The best way to a quick diagnosis in the first 2 weeks is PCR. Blood tests are only useful after 2 weeks of symptoms.

A clinical (without laboratory tests) diagnosis of whooping cough can only be made with hindsight. It can take 2 to 4 weeks before the diagnosis may become clear.

Some further general observations about the early symptomatic phase. 

Feverishness is unusual, but listlessness in the early stages seems quite common.

Sometimes the cough is worse in the day, or only at night, or when lying down. It varies tremendously from person to person. 

Many people do not have catarrh symptoms in my experience, although most descriptions of whooping cough describe it.

I believe that having a cough or cold makes people more susceptible to whooping cough. So people who turn out to have whooping cough have sometimes had a viral cough or cold before it. If that happens, the catarrh may be from the cold, not from the whooping cough. That makes for confusion because it will be impossible to know when the whooping cough actually started. It may give the impression that the early symptoms have been going on for more than 2 weeks. This situation happens quite a lot and makes sorting it out an almost impossible task for doctors and patients.

It is particularly difficult for asthmatics

It can be very confusing for asthmatics, who are quite used to coughing, sometimes with choking. But asthmatics are more susceptible to whooping cough and are likely to be undiagnosed for longer. There is a useful sign however. Asthmatics with whooping cough are usually aware that although the cough is a bit familiar, their chest is not tight as they would expect if it were asthma doing it. They usually recognise this but often do not think it important enough to mention.

Bottom line

Whooping cough cannot usually diagnosed until there has been at least 2 weeks of severe paroxysmal coughing. That is usually about 4 weeks from the start. Meantime it has very likely been passed to others.