#1. Take it to Autozone or Pep Boys and have the codes scanned.
#2. Write the codes down.
#3. Erase the codes (you have to do it, they can't legally)
#4. Google the CEL codes read in #1 above.
#4. Alternate: check the codes on this generic GM list I posted a while back:
There is no "diagnostic mode" The government required all manufactured to use the OBD-II system back in 1997, and it is still the industry standard for diagnostics.
FYI, you can by a bottom dollar Actron OBD-II scanner for about $60 that will read the codes, but like I said above, several placed will read them for free.