Large meteorites with the potential to cause major global environmental change strikes the earth on average every 100,000 years, according to NASA. Geologically speaking, 100,000 years is a short time interval making it plausible that every so often a "biggie" meteorite could hit the earth and cause a global catastrophe on the Permian type scale. When a large meteorite collides with the earth it leaves much more evidence than the obvious crater. Meteorite craters are often destroyed by burial or plate tectonics so for older impacts where the crater has long since gone the other lines of evidence assume more importance.
Earth elements occur in different proportions in meteorites than in the Earth's outer crust and one of the most important of these for meteorite strikes is Iridium. Iridium is a stable element which is only present in the Earth's crust in extremely tiny amounts but is present in much larger quantities in meteorites. Upon vaporization at impact, Iridium is scattered into the atmosphere and gradually falls onto the Earth's surface over time. Rocks that were at the surface at the time of impact are then enriched in iridium which can be measured today in a laboratory. Even relatively small meteorites can cause changes in iridium levels, producing so-called "iridium spikes", because there is so little originally in the crust.
Comets can also impact the Earth bit unfortunately they contain little or no Iridium.
Shocked quartz (or Stishovite) is a high pressure polymorph of quartz produced at or close to the impact site. It is produced under such high pressures that the only way it can form is by cosmic impacts. Finding traces of this in any geological strata is a very good indicator of an impact. Shocked quartz grains can only usually be seen with a scanning electron microscope.
Much rapid deposition occurs near to an impact site and the ejectile beds can often be preserved in the geological record. These horizons appear similar to others produced by other means so that they are difficult to identify as ejectile beds.
Meteorites have become a popularly debated cause for mass extinctions ever since the Chixulub crater was identified in the early 90's. The Chicxulub crater is in the Yukatan peninsular in Mexico and is seen as good evidence for a meteorite which could have killed off the dinosaurs in the K-T extinction event. This has been dated accurately to the K-T extinction event by radiometric techniques and also through stratigraphy analysis.
Since this discovery much work has been focused on meteorite impacts being possible causes of other mass extinctions with many people focusing on the largest, the Permo-Triassic event . Unfortunately the evidence for an impact at the time of the Permian / Triassic boundary is much more ambiguous that that for the Cretaceous-Tertiary boundary. There are many reasons for this with the best one probably being that the Permian was much longer ago with the evidence being exposed for much longer.