This may speak to some of your concerns:
American Safe Climbing Association
The American Safe Climbing Association publishes guidelines for safe bolting. Their stuff usually targets the people who are actually doing the bolting, but can be worth reading for general purposes. They have articles on how to tell which bolts are good and which are bad. And the have a lot of other interesting safely stuff
The list below is all fairly subjective, and you should still read the ASCA's website, but here's what I look for -
- if you're at a popular climbing area on an established climb, the rock for the bolts is probably fine.
- look for "choss" around the bolts - is the bolt attached to any rock that looks loose or broken, or sounds "hollow" when you thump it with your palm? A foot of solid rock on all sides of a bolt would normally seem totally solid.
- Look at the type of rock? Is it granite? Is it a bomber variety of sandstone? Or does the route generally have a lot of broken, hollow-sounding, or rattling holds that make noise as you climb on them (a bad sign), or does it feel like a giant, solid mass?
- is the bolt itself showing an unusual amount of rust
- double check that the bolt itself is reasonably tight. It doesn't have to be totally ratcheted down, but it should be right enough to more or less hold the hanger in place, and have enough threads sticking out the end that it couldn't unscrew while climbing.
- You can build a double sliding-X (with limiter knots) if you want to make sure you're distributing the load between both bolts. Use a pair of double-length slings. Put lockers into each bolt, and a pair of lockers (opposite and opposed) at the master point.
Bolts will sometimes fail, but not that often. The only time I've heard of it was on a new route when the route bolter wasn't finished with the route, but didn't adequately communicate it. He had a known-bad bolt on a route, and it blew when someone took a lead fall on it: Bolt Failure at New River Gorge
If you doubt the bolts, you could certainly rig an anchor (or a backup) off trees or boulders near the top of the route (if one is available), or just not do the climb. Trust your gut on this stuff - I think that bolts are generally safe, but don't know your area.