To add to the methods shared by others, one way I think is really cool is optically stimulated luminescence.

  So, a grain of quartz destined someday to become the perfect OSL sample will 
  begin its journey by emptying its electron traps. This typically happens 
  through steady sunlight exposure, in a process called bleaching. Bleaching 
  resets the OSL clock. In this example scenario, the grain is fully bleached 
  by the sun as the wind blows it across the landscape, until it finally 
  settles and additional wind-blown material buries it.
  
  Once it is cut off from light, the OSL clock begins running. The sediment now 
  surrounding the quartz will include tiny amounts of radioactive isotopes, 
  exposing the quartz to a steady flow of ionizing radiation. The quartz 
  captures electrons from this radiation; the radiation flow is called the dose 
  rate. This is like the steady ticking of a clock. The quartz begins trapping 
  electrons, and because it is cut off from light, the trapped electrons will 
  continue building up at a fairly steady, measurable rate.

Egyptologists can access science and have access to cultural artifacts and those artifacts include writing and that writing can be read and what can be read includes dates.