It is said that human hindsight is often sharper than human foresight.

Well, it seems that geologists have actually been  busy with the hindsight part: they've developed a way to estimate the size and collision frequency of meteors that have struck the earth.

Francois Paquay is one such geologist. The basis for his estimation is the use of variations (isotopes) of the rare element osmium. Sediments of these are found at the bottom of the ocean.

When meteorites collide with the planet, they carry different levels of osmium isotope than those normally found in the oceans. A meteorite collision causes osmium to mix throughout the ocean quickly. Deep sea sediments then preserve records of changes in the ocean chemistry. 

By comparing isotope levels before and after impact, geologists like Paquay can estimate the size of a particular meteor that crashed during a previous era. Paquay and his colleagues believe that the "osmium method" will complement the more well-known method of using iridium to estimate meteorite size and impact frequency.