The Dell Tag number is sometimes visible from the BIOS update page. Hit F2
when first booting up the Dell computer and this will take you into BIOS
update page.
If the Dell Tag number is actually on the BIOS page, you can also view it
by using a free utility called Belarc Advisor.
Legally download Belarc Advisor free from he
http://www.belarc.com/free_download.html
The Dell Tag number is usually pasted on the top of the computer near the
front cover, but I've also seen them pasted on the either side of the
computer case and on the back side of the older computers.