To add to a set of already great answers I would like to add two points: Glued bolts and weird old bolts.
There are generally two types of bolts used presently: Mechanical and glued bolts. Mechanical bolts are mostly sleeve anchors: When tightening the nut the first time a cone is driven under the sleeve which is thus forced against the wall of the borehole. This type of anchor is hard to test. In general it is very reliable but has one failure mode: If tightened too much, the holding strength decreases a lot, but usually not enough to just pull it out by hand when testing. So you just have to hope the route establisher did a good job (generally true, but accidents have happened).
Glued bolts have a smaller diameter than the borehole and are held in place by glue. This should be more corrosion resistant as it is waterproof, still the glue and bolt need to be corrosion resistant. One prominent failure mode of glued bolts can be easily tested. If the borehole was not properly cleaned before gluing, you can turn the bolt. Just insert your quickdraw and use the biner to try to turn the bolt. Still in frequented areas, I hardly ever do this. For a toprope anchor it is certainly not a bad idea to check this.
The second point are exotic/antique anchors. There are norms today, but you will still find gear that is not normed. Especially on old or alpine routes. The oldest kind are "Stift/Stich"-anchors (in German, do not now whether there is an English equivalent). These are the earliest hand drilled anchors. They are rectangular and quite fin, usually not very deep. If you find one consider yourself lucky and you may use it for commemoration, but they probably wont hold a fall. Apparently "Kronenbohrhaken" are the socalled spits @VladimirF mentioned in his answer. These came up in the 70s and could hold a fall, thus permitting a huge increase in sport climbing level. However they were hand drilled and had the drilling bit at the top of the bolt itself, so it is hardened steel. This is bad for corrosion and stability, so today they should not be used anymore. You will know one when you see it (rusted, antique looking design).
In the alps (I do not know about other regions) you can often find so called "Muniring". These can be huge as on the photo below or any other size. However they are not normed and you have no idea what kind of bolt is in the rock. I was once shown a removed "Muniring" of about the size of the one below that had a 6mm diameter about 2-3cm long bolt attached to it. So these are not reliable.
There is a plethora of other sometimes homemade anchors. You should never use those as your main protection - well, if it is your only protection, which does happen, you might as well :)
Image source: http://alpen.sac-cas.ch/fileadmin/data/flippingbook/SAC_Jahrbuch_2010_de/2010_03_d/files/assets/seo/page45.html