bring a piece of paper, a piece of aluminimum to the shop
then try to use the lens to cut those things, if it can cut both, it will be "sharp"
lameness aside
using lcd alone cannot tell, and frankly every lens has an indviidaul "sweet aperture" - usually around f/8 to f/11 will be fair enough if you ask me
for me i don't really care, lcd zoom in sharp enough can already
basically you should just use
a) lowest iso possible
b) put it on a solid base if tripod not available
c) use sweet aperture as mentioned above, if don't know, USUALLY f/8 should be ok, or 2 or 3 stops down from maximum aperture
d) zoom in to see lor