One stick at a time in the 1st DIMM slot.  If none works, try a known working stick in all of the DIMMS in case of bad DIMM(s).
Other than using Dell Diagnostics, what method would you use to check your systems RAM for problems? For more questions regarding this contest, please visit the Information Page here.
DS 07/22: How can you check your systems RAM for problems, without using Dell 
Diagnostics?