They use any of the method below to make the assumption:

1. Hypotheses from the inscription inside the tomb
2. Radiocarbon dated any organic material found inside the pyramid
3. date it by the pyramids' position in the development of Egyptian architecture and material culture over the broad sweep of 3,000 years

If you're really interested in how they do it, you should really read the interview in the source below, especially on the second page
Source(s):
http://www.pbs.org/wgbh/nova/pyramid/explore/howold.html

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