So what does a bad bolt look like? The following diagram shows a number of old bolt fixtures. It is unlikely that any of these ancient bolts or hangers would hold body weight...much less a fall.
Don't get lulled into a sense of complacency by shiny new-looking bolts, a small percentage of these are bad too. Some bolters use substandard equipment because it's cheaper. Others may place a bolt incorrectly. These are hard things to evaluate on the sharp end of the rope when your forearms are completely pumped out and you're about to whip.
There are three ways to evaluate a route with new-looking bolts prior to climbing it. First, look through the guidebook and see how many routes the first ascentionists put up. The more routes they have to their names, the less likely it is that they made a mistake bolting. Second, the more popular the route is, the more likely it is that the bolts have been evaluated by people who have placed a lot of bolts. Indeed, on a popular route it is also more likely that the bolts have held falls. And third, question the locals. If there is something amiss on a route, local climbers are usually aware of it.
People laugh when I tell them that I trust traditional gear more than I trust bolts. The reality is that with trad climbing I can always assess my own placements and I can always adjust them until they're perfect. There is little that I can do with a bolt that was placed by a stranger. And even less that I can do if it's thirty years old. That's not to say that I don't trust new-looking bolts. I do...but I do so with reservations.
(Diagram Credits -- "Bolts: Bomber or Time Bombs" by Todd Vogel. Rock and Ice #62, July 1994 -- Reprinted at the American Safe Climbing Association website.)