The precise method is to count the number and type of each chip (after looking them up in a databook for that DRAM manufacturer). However, you can get a good guess just by counting the number of chips.
DRAMs (for PC SIMMs) are either 1 or 4 bits wide. The total bit width is 8 or 9 (for 30 pin SIMMs) and 32 or 36 (for 72 pin SIMMs). DRAMs to hold parity are usually 1 bit wide to allow byte writes. Some examples:
2 chips: 8 bit (2x4bit) - no parity 3 chips: 9 bit (2x4bit + 1x1bit) - parity 8 chips: 8 bit (8x1bit) or 32 bit (8x4bit) - no parity 9 chips: 9 bit (9x1bit) - parity 12 chips: 36 bit (8x4bit + 4x1bit) - parity
Some new 72 pin SIMMs have two 32 (or 36) bit banks per SIMM and therefore have double the number of chips as a normal SIMM.
It also seems that some cheap SIMMs have begun using 'fake' parity on SIMMs; XOR gates that generate parity from 8 bit data rather than store and recall the actual parity generated by the DRAM controller. The only way to tell if you've been taken by one of these fake parity SIMMs is to look up all of the suspected parts in a DRAM databook.