This might work for some people, but for me it would have been a worthless and tragically misleading signal. I got modest-at-best enjoyment from being around kids before I had one, but the past 10 years with our son has held many of the best experiences of my life. Your own kid is a very different thing than all the other kids you might spend time with.
My experience is more like pg describes, where I had a lot of trouble projecting what it would be like, and in retrospect all the rational pro/con analysis I did wasn't really getting to the core of the matter.
I certainly didn't mean to suggest that you would enjoy spending time around a stranger's children as much as your own! I just mean that if you like spending time around a stranger's children more than average, or if you find babysitting less grating than average, this gives you some info that's specific to you, rather than being at the population level.
My experience is more like pg describes, where I had a lot of trouble projecting what it would be like, and in retrospect all the rational pro/con analysis I did wasn't really getting to the core of the matter.