Specs dont say really all that much. You can have a F2.8 lens but that does not really mean it performs any good at at F2.8. It also does not say all that much about the bokeh or how sharp your picture is or how much CA you have.
thats why I like to look at pictures from other people how the lens performs and I just look at the overal result of the photo.
Also look at the lens gallery we have on this forum with some excellent examples of what a lens can do (some even straight out of the camera with no photoshop tricks).
On good reviews they test the lens at different apertures to test center and corner sharpness. This is nice to know but since I dont photograph testcharts I also look at real life results from regular people.
It usually does not take me much time to figure out if a lens is the one i want or not. usually its all great sharp pictures from all users or its all soft pictures for example.
And then after that I gonna read about build quality of the lens. So far I always ended up with nikkor lenses.
I actually rather want a Analogue mid format Hasselblad, but thats another story....