Here's something I once remember copying off the ACORP site quite some time ago. I shall cut & paste it as is.
"Tip: Here is a trick to check if your DIMM is single-sided or double-sided -- if there are traces connected to golden finger pin 144 and pin 129 of the DIMM, the DIMM is probably double-sided; otherwise it is single-sided."
Note: the reason for this is that some DIMMs with, let's say, 4 chips on both sides may actually have all 8 chips hardwired to one single side, so a simple physical inspection isn't always enough.