My previous post #1666 outlined the general methods. You would have to check the web for individual methods used on specific fossils.
From #1666
Dating is done in several different ways. One is radiometric dating (of which there are several different kinds). Another deals with fauna and flora in associated strata. Still another deals with distinctive volcanic deposits. Using multiple methods helps to get reliable dates. But the dates are always cross-checked. If a fossil does not seem to fit, some bright person in some other facility will probably try to correct the date, and get a major research paper out of it.
Your the one who presented this as some kind of fact - don't you think you should know what dating method or methods were used?