You've probably asked one of the more pertinant and well thought out questions to appear on these forums.
Actually, a lot of people don't bother or think to see if they have the apptitude to fly. Often getting far through an atpl course and just finding they haven't got the ability to be a good commercial pilot. Sometimes this doesn't even show up until they are doing sim tests with airlines.
Thankfully some of the bigger FTOs include some sort of apptitude testing before taking you on, but there are still a large number that don't and even those that do can sometmes be more motivated by taking your money.
At the very least you should consider taking a trial flight. Pop down to your local airfield and get a flight for an hour and see if you actually like flying and enjoy it.
However, even that won't give you a great indication of apptitude and suitability to be a commercial pilot. A wise thing to do, and was recommended at the recent PPRuNe seminar is to take the GAPAN apptitude tests. At £150, they could be the wisest investment you make compared to the many thousands and atpl costs!
http://www.gapan.org/careers/aptitude.htm