How can you prove that this earth has been around for millions of years?

Scientists have been studying and because of the big bang they were able to determine how long earth has been around. Also, ever heard of dinosaurs? When they found fossils they were able to look at the imprint they made and yeah...

Actually, most scientists agree that the Earth has been around for about 4.5 billion years. They know this because of the fossil and geological records, but primarily because they have been able to analyze radioactive decay in comparision with the expected half-life of the various radioactive isotopes that exist today.