How do I tell the difference between dual and single rank modules?

Multiply total number of chips (less any ECC chips)  by the chip width. If it equals 64 its single rank. If it equals 128 its dual rank.
Example:
D1240SR002GE431804
number of chips less ECC chips = 16
DRAM width is x04
16 x 4 = 64
This is a single-rank module.
D1240SR002GE433604
number of chips less ECC chips = 32
DRAM width is x04
32 x 4 = 128
This is a dual-rank module.
This information is important on many server applications as there are usually limitations on maximum number of ranks supported.  Also, some servers will only support dual-rank modules in certain memory sockets.