Through experience, with help from your local bike shop, or via a fit calculator like the one at Competitive Cyclist, you should be able to get a ballpark understanding of what'll fit, and you can adjust your contact points to make it work. So long as you get reasonably close on the frame size, you should be able get your fit dialed in through adjustments (such as saddle height and angle) and component selection (handlebar stem length and angle).

This could be a sign of a physiological issue like nerve compression, beyond the scope of basic bike fitting. You may want to ask your doctor for his advice. Are you having numbness in the thumb and first two fingers or that half of the palm, which might indicate carpal tunnel syndrome? Or maybe the ring finger & pinky, which might hint at ulnar nerve compression?

With that said, there are things you can try to alleviate numbness: Try a different type of handlebar or grip so you're not putting pressure on the same points. Change your hand position frequently as you ride. Make sure you're not putting too much weight on your hands, which may require adjustments to saddle & handlebar position. Try padded cycling gloves and/or fatter, more padded grips.

I can pretty quickly get a feel for whether the frame size is right. Is the bike too tall for me? Top tube too long or too short for me? If not, everything else is adjustable. I know I'll be tweaking things after the first ten (or hundred) miles to get things dialed in perfectly. At that point, it's not a matter of buying the bike that fits right, but adjusting your bike so that it fits just right.