Here's how you do it: use a small flashlight (LEDs are brighter):
Start at the top of the tire above the tire valve (to keep track of where you are). Slowly examine your tire for anything unusual. Anything white. Any changes in the surface. Glass will usually sparkle. You'll be surprised at how much glass you are carrying. A lot of it can just be brushed off. But look again for irregularities. What you'll often find is a piece of glass embedded in a crevice. Sometimes you can't spot it right off, but if you investigate carefully with your "point," (kind of like a dentist would actually), you'll actually feel it. See if you can pick it out. Be careful - too much digging will result in the very puncture you're doing this to avoid. If that glass is really stubborn, it's better to deflate the tire. Then squeeze the tire, and it will pop right out.
Because glass is guilty. It has no business in your tires, and is quick to admit it. (incidentally, gravel can be just as bad and if you ride on it, the same concept applies).
The more you do this, the better you'll get at it. And it's especially rewarding when you find a piece of glass or gravel that you know would have spelled certain disaster on the road; an obvious shard just hanging out in your tire. What - do you want to be caught? (Yes).
The guys I know at Zen say they do this once a week, and even deflate their tires to be thorough. I check my tires every couple of days, because Lola's tires are smooth, more vulnerable than tires with tread. And riding to the Hudson Greenway there are night clubs (broken bottles), construction areas, and too often, a car window break-in - hey this is New Yawk.
You especially want to do this before or after riding on wet streets. If there's any glass in your tires, water will make it easier for it to travel to the tube and puncture it.
Flats can happen to anybody. And if you get a flat, you're going to have to dig that glass out anyway. So why not reduce the chances? Because how would I rather spend my time: picking my tires at home, or changing a flat on the road?