The best sign that an egg has been laid will be when one of the eagles suddenly begins incubating. Remember that, unlike many other bird species, incubation begins when the first egg is deposited, causing asynchronous hatching. Usually it will be the female that sits on the eggs the first day or so, but they may switch duties. In any event one bird will always be sitting deep on the nest after the first egg is laid. That observation has proven to be an accurate way to begin counting the incubation days for the past two nesting seasons.
Observers saw this happen on December 13, 2008 and the first egg hatched January 17, 2009. Then the next season the female began sitting low on December 18, 2009 and the first egg hatched on January 22, 2010. Both hatched exactly 35 days after the observed onset of incubation.
Great observations, Jill. Be careful on that car roof! Maybe we can rent one of those double deck buses from London.